refactor(calendar): stack layout, wider grid, fixed cell height

- Calendar widget stays stacked (calendar above events list)
- Grid widens to max-w-2xl with fixed h-10 cells (no aspect-square height growth)
- Gap between cells: 0.5 → 1.5
- Button system extracted to shared btn-* utility classes in app.css
- Replaced Google Calendar link + Google Maps with OSM
- Removed google icon from iconify subset

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

src/lib/translations.ts

@@ -121,7 +121,6 @@ const TRANSLATION_KEYS = [
   "calendar_no_future_events",
   "calendar_clear_filter",
   "calendar_event_count",            // {{n}}
-  "calendar_add_to_google",
   "calendar_download_ics",
   "calendar_open_maps",
   "calendar_show_past",