feat: full Discord-style presence tracking

- Announce online immediately on app startup
- Idle detection: unavailable after 10 min of no input, online on return
- Tab visibility: unavailable when hidden, online when focused again
- Page close: offline via fetch+keepalive (survives unload without bfcache penalty)
- hidePresence setting: broadcasts offline and stops all tracking
- Added 'Hide Online Status' toggle in General settings

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

src/app/state/settings.ts

@@ -44,6 +44,7 @@ export interface Settings {
   twitterEmoji: boolean;
   pageZoom: number;
   hideActivity: boolean;
+  hidePresence: boolean;
 
   isPeopleDrawer: boolean;
   memberSortFilterIndex: number;
@@ -87,6 +88,7 @@ const defaultSettings: Settings = {
   twitterEmoji: false,
   pageZoom: 100,
   hideActivity: false,
+  hidePresence: false,
 
   isPeopleDrawer: true,
   memberSortFilterIndex: 0,