feat(settings): autosave on change, remove Save buttons (closes #128)

Each setting now saves via a debounced (600ms) individual API call when
its value changes. A hydration guard (_hydrated flag + nextTick) prevents
watchers from firing during the initial load() fetch, ensuring the first
API round-trip does not generate spurious write calls.

Removed: five explicit Save buttons across Equipment, Sensory, Units,
Shopping Region, and Recipe Search Layout sections.
Added: "Changes save automatically." subtitle + fixed bottom-right toast
  that appears for 2s after any successful save, with enter/leave
  transitions that respect prefers-reduced-motion via the theme.

The full save() and saveSensory() actions are kept as internal fallbacks.

frontend/src/components/SettingsView.vue

@@ -2,6 +2,7 @@
   <div class="settings-view">
     <div class="card">
       <h2 class="section-title text-xl mb-md">Settings</h2>
+      <p class="text-xs text-muted mb-md">Changes save automatically.</p>
 
       <!-- Cooking Equipment -->
       <section>
@@ -50,18 +51,6 @@
           </div>
         </div>
 
-        <!-- Save button -->
-        <div class="flex-start gap-sm">
-          <button
-            class="btn btn-primary"
-            :disabled="settingsStore.loading"
-            @click="settingsStore.save()"
-          >
-            <span v-if="settingsStore.loading">Saving…</span>
-            <span v-else-if="settingsStore.saved">✓ Saved!</span>
-            <span v-else>Save Settings</span>
-          </button>
-        </div>
       </section>
 
       <!-- Sensory Preferences -->
@@ -134,17 +123,6 @@
           </p>
         </div>
 
-        <div class="flex-start gap-sm mt-sm">
-          <button
-            class="btn btn-primary btn-sm"
-            :disabled="settingsStore.loading"
-            @click="settingsStore.saveSensory()"
-          >
-            <span v-if="settingsStore.loading">Saving…</span>
-            <span v-else-if="settingsStore.saved">Saved!</span>
-            <span v-else>Save sensory preferences</span>
-          </button>
-        </div>
       </section>
 
       <!-- Units -->
@@ -169,17 +147,6 @@
             Imperial (oz, cups, °F)
           </button>
         </div>
-        <div class="flex-start gap-sm">
-          <button
-            class="btn btn-primary btn-sm"
-            :disabled="settingsStore.loading"
-            @click="settingsStore.save()"
-          >
-            <span v-if="settingsStore.loading">Saving…</span>
-            <span v-else-if="settingsStore.saved">✓ Saved!</span>
-            <span v-else>Save</span>
-          </button>
-        </div>
       </section>
 
       <!-- Shopping Locale -->
@@ -220,17 +187,6 @@
             <option value="br">Brazil (BRL R$)</option>
           </optgroup>
         </select>
-        <div class="flex-start gap-sm mt-sm">
-          <button
-            class="btn btn-primary btn-sm"
-            :disabled="settingsStore.loading"
-            @click="settingsStore.save()"
-          >
-            <span v-if="settingsStore.loading">Saving…</span>
-            <span v-else-if="settingsStore.saved">✓ Saved!</span>
-            <span v-else>Save</span>
-          </button>
-        </div>
       </section>
 
       <!-- Time-First Layout -->
@@ -258,17 +214,6 @@
             </span>
           </label>
         </div>
-        <div class="flex-start gap-sm mt-sm">
-          <button
-            class="btn btn-primary btn-sm"
-            :disabled="settingsStore.loading"
-            @click="settingsStore.save()"
-          >
-            <span v-if="settingsStore.loading">Saving…</span>
-            <span v-else-if="settingsStore.saved">✓ Saved!</span>
-            <span v-else>Save</span>
-          </button>
-        </div>
       </section>
 
       <!-- Data Sharing (cloud only) -->
@@ -393,6 +338,12 @@
       </template>
     </div>
   </div>
+
+  <Transition name="autosave-fade">
+    <div v-if="settingsStore.saved" class="autosave-toast" role="status" aria-live="polite">
+      ✓ Saved
+    </div>
+  </Transition>
 </template>
 
 <script setup lang="ts">
@@ -871,4 +822,32 @@ function getNoiseClass(_value: NoiseLevel, idx: number): string {
   border-color: var(--color-border, #e0e0e0);
   color: var(--color-text-secondary, #888);
 }
+
+/* ── Autosave toast ──────────────────────────────────────────────────────── */
+
+.autosave-toast {
+  position: fixed;
+  bottom: 1.5rem;
+  right: 1.5rem;
+  background: var(--color-surface, #fff);
+  border: 1px solid var(--color-border, #e0e0e0);
+  border-radius: var(--radius-md, 0.5rem);
+  padding: 0.4rem 0.9rem;
+  font-size: var(--font-size-sm);
+  color: var(--color-success, #4a8c40);
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.12);
+  z-index: 500;
+  pointer-events: none;
+}
+
+.autosave-fade-enter-active,
+.autosave-fade-leave-active {
+  transition: opacity 0.25s ease, transform 0.25s ease;
+}
+
+.autosave-fade-enter-from,
+.autosave-fade-leave-to {
+  opacity: 0;
+  transform: translateY(0.5rem);
+}
 </style>

frontend/src/stores/settings.ts

@@ -1,11 +1,5 @@
-/**
- * Settings Store
- *
- * Manages user settings (cooking equipment, preferences) using Pinia.
- */
-
 import { defineStore } from 'pinia'
-import { ref } from 'vue'
+import { ref, watch, nextTick } from 'vue'
 import { settingsAPI } from '../services/api'
 import type { UnitSystem } from '../utils/units'
 import type { SensoryPreferences } from '../services/api'
@@ -13,8 +7,12 @@ import { DEFAULT_SENSORY_PREFERENCES } from '../services/api'
 
 export type TimeFirstLayout = 'auto' | 'time_first' | 'normal'
 
+function debounce(fn: () => void, ms: number): () => void {
+  let t: ReturnType<typeof setTimeout>
+  return () => { clearTimeout(t); t = setTimeout(fn, ms) }
+}
+
 export const useSettingsStore = defineStore('settings', () => {
-  // State
   const cookingEquipment = ref<string[]>([])
   const unitSystem = ref<UnitSystem>('metric')
   const shoppingLocale = ref<string>('us')
@@ -23,7 +21,40 @@ export const useSettingsStore = defineStore('settings', () => {
   const loading = ref(false)
   const saved = ref(false)
 
-  // Actions
+  // Prevents autosave watchers from firing during initial load hydration.
+  // Set to true after nextTick() at the end of load() — by that point all
+  // watcher jobs queued by the hydration assignments have already flushed.
+  let _hydrated = false
+
+  function _flash() {
+    saved.value = true
+    setTimeout(() => { saved.value = false }, 2000)
+  }
+
+  async function _saveKey(key: string, value: string): Promise<void> {
+    if (!_hydrated) return
+    try {
+      await settingsAPI.setSetting(key, value)
+      _flash()
+    } catch (err: unknown) {
+      console.error('Autosave failed for key:', key, err)
+    }
+  }
+
+  const _autosave = {
+    equipment: debounce(() => _saveKey('cooking_equipment', JSON.stringify(cookingEquipment.value)), 600),
+    unit:      debounce(() => _saveKey('unit_system', unitSystem.value), 600),
+    locale:    debounce(() => _saveKey('shopping_locale', shoppingLocale.value), 600),
+    sensory:   debounce(() => _saveKey('sensory_preferences', JSON.stringify(sensoryPreferences.value)), 600),
+    layout:    debounce(() => _saveKey('time_first_layout', timeFirstLayout.value), 600),
+  }
+
+  watch(cookingEquipment, _autosave.equipment, { deep: true })
+  watch(unitSystem, _autosave.unit)
+  watch(shoppingLocale, _autosave.locale)
+  watch(sensoryPreferences, _autosave.sensory, { deep: true })
+  watch(timeFirstLayout, _autosave.layout)
+
   async function load() {
     loading.value = true
     try {
@@ -58,8 +89,15 @@ export const useSettingsStore = defineStore('settings', () => {
     } finally {
       loading.value = false
     }
+    // Yield past the watcher flush triggered by hydration assignments above.
+    // After nextTick, any pending watcher jobs from this load() have already
+    // run (and been ignored by _hydrated guard), so user-driven changes from
+    // here forward will correctly trigger autosave.
+    await nextTick()
+    _hydrated = true
   }
 
+  // Kept for explicit full-save scenarios (e.g. fallback, tests).
   async function save() {
     loading.value = true
     try {
@@ -70,10 +108,7 @@ export const useSettingsStore = defineStore('settings', () => {
         settingsAPI.setSetting('sensory_preferences', JSON.stringify(sensoryPreferences.value)),
         settingsAPI.setSetting('time_first_layout', timeFirstLayout.value),
       ])
-      saved.value = true
+      _flash()
-      setTimeout(() => {
-        saved.value = false
-      }, 2000)
     } catch (err: unknown) {
       console.error('Failed to save settings:', err)
     } finally {
@@ -81,24 +116,17 @@ export const useSettingsStore = defineStore('settings', () => {
     }
   }
 
+  // Kept for backward compat; autosave handles sensory changes now.
   async function saveSensory() {
-    loading.value = true
     try {
-      await settingsAPI.setSetting(
+      await settingsAPI.setSetting('sensory_preferences', JSON.stringify(sensoryPreferences.value))
-        'sensory_preferences',
+      _flash()
-        JSON.stringify(sensoryPreferences.value),
-      )
-      saved.value = true
-      setTimeout(() => { saved.value = false }, 2000)
     } catch (err: unknown) {
       console.error('Failed to save sensory preferences:', err)
-    } finally {
-      loading.value = false
     }
   }
 
   return {
-    // State
     cookingEquipment,
     unitSystem,
     shoppingLocale,
@@ -106,8 +134,6 @@ export const useSettingsStore = defineStore('settings', () => {
     timeFirstLayout,
     loading,
     saved,
-
-    // Actions
     load,
     save,
     saveSensory,