Redesign watchface with countdown bar and calendar support

- Force 24h time format; switch date to DD/MM (Gothic 28 Bold)
- Replace effect-layer invert with explicit black background + white text
- Add 8px top bar counting down to next :00/:30; preempts to nearest
  calendar appointment when one arrives via AppMessage within 30 min
- Add pkjs with Google Calendar API integration (falls back gracefully
  if no API key is configured)
- Move battery indicator to a 4px full-width strip at screen bottom
- Drop pebble-effect-layer dependency; add APPOINTMENT_MINUTES messageKey
- Write README with feature overview, build instructions, and calendar setup

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

countdown_watchface/package.json

@@ -6,9 +6,7 @@
     "pebble-app"
   ],
   "private": true,
-  "dependencies": {
-    "pebble-effect-layer": "^1.2.0"
-  },
+  "dependencies": {},
   "pebble": {
     "displayName": "countdown_watchface",
     "uuid": "769d0058-ae41-4e80-b41a-64d58e2ae3e0",
@@ -25,7 +23,7 @@
       "watchface": true
     },
     "messageKeys": [
-      "dummy"
+      "APPOINTMENT_MINUTES"
     ],
     "resources": {
       "media": []