Add map and calendar views (Phase 5)

Map view: Leaflet + OpenStreetMap with markers from NIP-52 geohash
tags. Click popup navigates to activity detail. Auto-fits bounds to
show all markers. Geohash decoding via ngeohash added to Activity
model conversion.

Calendar view: Month grid with activity dots (up to 3 per day).
Click a day to see activities listed below the calendar. Month
navigation with prev/next buttons.

New dependencies: leaflet, @types/leaflet, ngeohash, @types/ngeohash.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -30,8 +30,10 @@
     "date-fns": "^4.1.0",
     "electron-squirrel-startup": "^1.0.1",
     "fuse.js": "^7.0.0",
+    "leaflet": "^1.9.4",
     "light-bolt11-decoder": "^3.2.0",
     "lucide-vue-next": "^0.474.0",
+    "ngeohash": "^0.6.3",
     "nostr-tools": "^2.10.4",
     "pinia": "^2.3.1",
     "qr-scanner": "^1.4.2",
@@ -60,6 +62,8 @@
     "@tailwindcss/forms": "^0.5.10",
     "@tailwindcss/typography": "^0.5.16",
     "@tailwindcss/vite": "^4.0.12",
+    "@types/leaflet": "^1.9.21",
+    "@types/ngeohash": "^0.6.8",
     "@types/node": "^22.18.1",
     "@types/qrcode": "^1.5.5",
     "@types/rollup-plugin-visualizer": "^4.2.3",