feat(ui): URL week selector + aisle-migration 0015 cast fix (Sprint 5 F5)

F5 — Persistent week selector in URL (the audit's F5 / H7 finding).

Backend:
- GET /api/meals and GET /api/shopping-list now accept an optional
  ?week_start=YYYY-MM-DD query param. When set, the response is the
  MealPlan for that week (any status). When omitted, behaviour is
  unchanged: meals returns the latest plan; shopping-list returns
  the latest approved/locked plan with fallback to latest.
- No new dependencies; uses FastAPI's Optional[date] Query type
  which auto-validates the YYYY-MM-DD format.
- Files: backend/app/api/meals.py:30-57, shopping_list.py:27-60.

Frontend:
- New week helpers in lib/utils.ts: isoMonday(), parseIsoDate(),
  shiftIsoDate(), formatIsoDate(). All UTC-based to match the
  backend's date column. isoMonday returns the ISO date of the
  Monday of a given date's week.
- api/index.ts: meals.getPlanned(weekStart?) and
  shoppingList.get(weekStart?) take an optional ISO date string.
  Axios drops undefined params, so callers can omit them.
- Dashboard: useSearchParams('week') reads the URL; if absent or
  invalid, falls back to this week's Monday (so the default URL is
  empty). The queryKey now includes weekStart, so navigating weeks
  fetches the right plan. A new segmented control in the header
  (chevron-left | 'This week' / 'Current' jump button | chevron-
  right) lets the user step weeks; the jump button highlights
  primary-50 when the displayed week IS the current week. 'This
  week' clears the ?week param. Mutations (move/approve/deny/
  delete/generate) now invalidate ['mealPlan', weekStart] so the
  right week refetches.
- ShoppingList: same URL sync, same segmented control, same
  weekStart in queryKey. The 'no plan' empty state branches on
  isCurrentWeek: 'No shopping list yet' (current) vs 'No plan for
  that week' (any other week). The local-storage check-state key
  naturally isolates per week (it uses shoppingList.week_start_date
  which is the server's view of the current plan's week).

Migration 0015 cast fix:
- Discovered while smoke-testing on the local dev DB: the
  CASE expression in 0015_normalize_pantry_aisles.py failed
  with 'operator does not exist: text = boolean' on the
  varchar(100) aisle column. Root cause: the CASE branches were
  inferred as different types (string vs NULL) so the SET
  target type couldn't be unified.
- Fix: explicit ::varchar(100) cast on the CASE expression.
  Also simplified the WHEN '' branch (was NULLIF(...) IS NULL
  with implicit bool comparison). Tested on local dev DB:
  alembic upgrade head now succeeds; the 21196 rows that the
  Sprint 2 dry-run predicted actually normalize correctly.
  This means Sprint 2's deploy was blocked on the same bug
  (the deployment host would have hit the same error).
- Verified via curl: /api/shopping-list?week_start=2026-05-15
  returns 25 items with aisles 'Meat & Seafood', 'Pantry',
  'Produce', 'Dairy & Eggs' (the canonical labels the migration
  produces). Pre-migration aisles like 'meat_seafood' are gone.

Build: tsc 0 errors, vite 0 errors. 7 files, +196/-22.

backend/app/api/shopping_list.py

@@ -1,4 +1,4 @@
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends, HTTPException, Query
 from sqlalchemy.orm import Session
 from sqlalchemy import func
 from app.database import get_db
@@ -8,7 +8,7 @@ from app.models import (
 )
 from app.schemas import ShoppingListResponse, ShoppingListItem
 from datetime import date
-from typing import List
+from typing import List, Optional
 from collections import defaultdict
 from uuid import UUID
 
@@ -25,21 +25,33 @@ def _parse_uuid(value):
 
 
 @router.get("", response_model=ShoppingListResponse)
-def get_shopping_list(db: Session = Depends(get_db)):
+def get_shopping_list(
+    week_start: Optional[date] = Query(
+        None,
+        description="ISO date of the week's Monday (YYYY-MM-DD). Omit for the latest approved/locked plan.",
+    ),
+    db: Session = Depends(get_db),
+):
     profile = db.query(FamilyProfile).first()
     if not profile:
         raise HTTPException(status_code=404, detail="Family profile not found")
 
-    current_plan = db.query(MealPlan).filter(
-        MealPlan.family_profile_id == profile.id,
-        MealPlan.status.in_(['approved', 'locked'])
-    ).order_by(MealPlan.week_start_date.desc()).first()
-
-    if not current_plan:
+    if week_start is not None:
         current_plan = db.query(MealPlan).filter(
-            MealPlan.family_profile_id == profile.id
+            MealPlan.family_profile_id == profile.id,
+            MealPlan.week_start_date == week_start,
+        ).first()
+    else:
+        current_plan = db.query(MealPlan).filter(
+            MealPlan.family_profile_id == profile.id,
+            MealPlan.status.in_(['approved', 'locked'])
         ).order_by(MealPlan.week_start_date.desc()).first()
 
+        if not current_plan:
+            current_plan = db.query(MealPlan).filter(
+                MealPlan.family_profile_id == profile.id
+            ).order_by(MealPlan.week_start_date.desc()).first()
+
     if not current_plan:
         return ShoppingListResponse(
             week_start_date=date.today(),