fix: meal planner auto-selects current week on load, + New week idempotent

- Add autoSelectPlan() to the store: after loadPlans() resolves, set
  activePlan to the current week's plan (or most recent) without a second
  API round-trip -- list already returns full PlanSummary with slots
- Call autoSelectPlan(mondayOfCurrentWeek()) in onMounted so the grid
  populates immediately without the user touching the dropdown
- Make onNewPlan idempotent: if a 409 comes back, activate the existing
  plan for that week instead of surfacing an error to the user

frontend/src/components/MealPlanView.vue

@@ -101,7 +101,10 @@ const canAddMealType = computed(() =>
   (activePlan.value?.meal_types.length ?? 0) < 4
 )
 
-onMounted(() => store.loadPlans())
+onMounted(async () => {
+  await store.loadPlans()
+  store.autoSelectPlan(mondayOfCurrentWeek())
+})
 
 function mondayOfCurrentWeek(): string {
   const today = new Date()
@@ -116,15 +119,20 @@ function mondayOfCurrentWeek(): string {
 async function onNewPlan() {
   planError.value = null
   planCreating.value = true
+  const weekStart = mondayOfCurrentWeek()
   try {
-    const weekStart = mondayOfCurrentWeek()
     await store.createPlan(weekStart, ['dinner'])
   } catch (err: unknown) {
     const msg = err instanceof Error ? err.message : String(err)
-    // 409 means a plan for this week already exists — surface it helpfully
+    if (msg.includes('409') || msg.toLowerCase().includes('already exists')) {
-    planError.value = msg.includes('409') || msg.toLowerCase().includes('already exists')
+      // Plan for this week exists — just activate it instead of erroring
-      ? 'A plan for this week already exists. Select it from the dropdown above.'
+      const existing = plans.value.find(p => p.week_start === weekStart)
-      : `Couldn't create plan: ${msg}`
+      if (existing) {
+        await store.setActivePlan(existing.id)
+      }
+    } else {
+      planError.value = `Couldn't create plan: ${msg}`
+    }
   } finally {
     planCreating.value = false
   }

frontend/src/stores/mealPlan.ts

@@ -40,6 +40,20 @@ export const useMealPlanStore = defineStore('mealPlan', () => {
     }
   }
 
+  /**
+   * Auto-select the best available plan without a round-trip.
+   * Prefers the plan whose week_start matches preferredWeekStart (current week's Monday).
+   * Falls back to the first plan in the list (most recent, since list is DESC).
+   * No-ops if a plan is already active or no plans exist.
+   */
+  function autoSelectPlan(preferredWeekStart?: string) {
+    if (activePlan.value || plans.value.length === 0) return
+    const match = preferredWeekStart
+      ? (plans.value.find(p => p.week_start === preferredWeekStart) ?? plans.value[0])
+      : plans.value[0]
+    if (match) activePlan.value = match ?? null
+  }
+
   async function createPlan(weekStart: string, mealTypes: string[]): Promise<MealPlan> {
     const plan = await mealPlanAPI.create(weekStart, mealTypes)
     plans.value = [plan, ...plans.value]
@@ -129,7 +143,7 @@ export const useMealPlanStore = defineStore('mealPlan', () => {
   return {
     plans, activePlan, shoppingList, prepSession,
     loading, shoppingListLoading, prepLoading, slots,
-    getSlot, loadPlans, createPlan, setActivePlan,
+    getSlot, loadPlans, autoSelectPlan, createPlan, setActivePlan,
     upsertSlot, clearSlot, loadShoppingList, loadPrepSession, updatePrepTask,
   }
 })