docs(ui): record backlog board baseline

Codex Documentation/Current Software Plan/UI Plan 2 - Backlog Board Implementation/UI_PLAN_2_IMPLEMENTATION_NOTES.md

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+<!-- SPDX-FileCopyrightText: 2026 FocusFlow contributors -->
+<!-- SPDX-License-Identifier: AGPL-3.0-only -->
+
+# UI Plan 2 Implementation Notes
+
+## Block 01 Baseline Map
+
+1. Sidebar and app shell currently render through
+   `apps/focus_flow_flutter/lib/widgets/sidebar.dart`,
+   `apps/focus_flow_flutter/lib/widgets/app_shell.dart`, and the
+   `FocusFlowHome` parts under `apps/focus_flow_flutter/lib/app/home/`.
+2. Navigation state is not implemented yet. `Sidebar` hard-codes Today as
+   active, and `FocusFlowHome` owns only Today controllers. UI Plan 2 should add
+   app-level section state in the home/composition layer before rendering the
+   Backlog board.
+3. Backlog board data needs new public scheduler application read models. The
+   current `BacklogQueryResult` is list-shaped and returns raw `Task` values via
+   `BacklogItemReadModel`, which is not enough for board columns, summary,
+   detail drawer metadata, or suggested slot previews.
+4. Existing commands include quick capture to backlog, schedule backlog item to
+   next available slot, move flexible tasks to backlog, remove task, and break
+   up task. Push-to-Someday and non-destructive backlog archive still need
+   command-level decisions before the drawer actions can be final.
+5. UI Plan 1 components to reuse include `FocusFlowTokens`, the dark shell,
+   timeline reward icon, difficulty bars, selected overlay patterns, and the
+   top-bar icon/segmented-control styling conventions.
+
+## Block 01 Validation
+
+- `scripts/bootstrap_dev.sh`: passed.
+- `dart analyze`: passed.
+- `dart test`: passed.
+- `flutter analyze` in `apps/focus_flow_flutter`: passed.
+- `flutter test` in `apps/focus_flow_flutter`: passed.