feat: export meal plan to a calendar (.ics) file

Hand-rolled a minimal RFC 5545 writer (lib/ics.ts) rather than adding
a dependency for what's a handful of VEVENTs. Meal slots have no
stored time-of-day, so each mealType maps to a conventional wall-clock
time (dinner 7pm, etc.) written as floating local time, not pinned to
a timezone. Authenticated-only download for now, not a subscribe URL
— that would need a new unguessable-link mechanism on meal plans,
which don't have one today (unlike shopping lists' isPublic).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 All notable changes to Epicure are documented here. This file is mirrored in-app at `/changelog` (and in the admin dashboard) via `apps/web/lib/changelog.ts` — update both together.
 
+## 0.19.0 — 2026-07-13 22:21
+
+### Added
+- **Export your meal plan to a calendar file** — download a week's meals as a .ics file to import into Google/Apple/Outlook calendar.
+
 ## 0.18.0 — 2026-07-13 22:16
 
 ### Added