feat: PiP mute indicator, export history, activity log, unverified device warning

- PiP call window: mute overlay using MutationObserver on EC iframe's
  [data-testid="incall_mute"] button (data-kind="primary" = muted),
  same pattern as screenshare detection in CallControl.ts

- P2-4 Export Room History: new tab in room settings — Plain Text / JSON /
  HTML formats, optional date range, progress counter, paginated via
  paginateEventTimeline, blob download; E2EE-aware (skips failed decryptions)

- P2-6 Room Activity Log: new tab in room settings — filterable log of
  m.room.member, m.room.power_levels, m.room.name/topic/avatar/server_acl
  events with human-readable descriptions, relative timestamps, Load More
  pagination

- P2-10 Unverified Device Warning: warnOnUnverifiedDevices setting (default
  off); Warning.Container banner above composer in encrypted rooms with
  unverified devices; toggle in Settings → General → Privacy

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

src/app/state/settings.ts

@@ -90,6 +90,8 @@ export interface Settings {
   nightLightOpacity: number;
 
   deafenKey: string;
+
+  warnOnUnverifiedDevices: boolean;
 }
 
 const defaultSettings: Settings = {
@@ -149,6 +151,8 @@ const defaultSettings: Settings = {
   nightLightOpacity: 30,
 
   deafenKey: 'KeyM',
+
+  warnOnUnverifiedDevices: false,
 };
 
 export const getSettings = (): Settings => {