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>

apps/web/messages/en.json

@@ -830,6 +830,7 @@
     "markCooked": "Mark as cooked",
     "markCookedSuccess": "Marked as cooked",
     "markCookedFailed": "Failed to mark as cooked",
+    "exportCalendar": "Export to calendar",
     "clearDay": "Clear day",
     "clearWeek": "Clear week",
     "clearDayConfirmTitle": "Clear {day}?",