feat: meals drag & drop between slots and days (BL-03)

Pointer Events-based drag & drop (touch + mouse compatible):
- Ghost element follows pointer; drops on empty slots move the meal,
  drops on occupied slots swap both meals via concurrent PUT requests
- prefers-reduced-motion: no ghost animation, interaction still works
- Suppress-click guard prevents accidental edit modal after drag

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Calendar: recurring events are now expanded in GET /api/v1/calendar — all occurrences within the requested date window are returned as virtual instances; duration is preserved; instances are marked with is_recurring_instance=1 and shown with a ↻ icon in the agenda view; /upcoming also expands recurring events within a 90-day window
 - Budget: recurring entries auto-generate instances for each viewed month; instances deleted by the user are skipped permanently via `budget_recurrence_skipped` table; generated instances are marked with ↩ in the transaction list
 - Budget: month-over-month comparison in summary cards — each card (Einnahmen, Ausgaben, Saldo) shows a trend line (▲/▼ + delta amount vs. previous month); previous month summary is fetched in parallel with current month
+- Meals: drag & drop between slots and days using Pointer Events (touch + mouse); ghost element follows pointer; drop on occupied slot swaps meals; reduced-motion: no ghost animation, interaction still works
 
 ## [0.2.1] - 2026-03-30