feat: add theme system with light/dark mode support

- Add ThemeStore (Zustand) for reactive theme switching
- Add Themes.tsx with THEMES object (defaultLight, defaultDark)
- Add Settings screen with theme switcher and logout button
- Add BaseButton component for reusable themed buttons
- Migrate all components from static currentTheme to useThemeStore()
- Add shadowColor to theme (iOS only, Android uses elevation)
- All text elements now use theme colors (textPrimary, textSecondary, etc.)
- Update tab navigation to include Settings tab
- Move logout from Header to Settings screen

apps/client/app.json

@@ -2,7 +2,7 @@
   "expo": {
     "jsEngine": "hermes",
     "name": "CalChat",
-    "slug": "calchat",
+    "slug": "caldav",
     "version": "1.0.0",
     "orientation": "portrait",
     "scheme": "calchat",
@@ -32,6 +32,7 @@
       "eas": {
         "projectId": "b722dde6-7d89-48ff-9095-e007e7c7da87"
       }
-    }
+    },
+    "owner": "gilmour109"
   }
 }