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/features/settings/general/General.tsx

@@ -865,6 +865,10 @@ function Privacy() {
     settingsAtom,
     'privateReadReceipts',
   );
+  const [warnOnUnverifiedDevices, setWarnOnUnverifiedDevices] = useSetting(
+    settingsAtom,
+    'warnOnUnverifiedDevices',
+  );
 
   return (
     <Box direction="Column" gap="100">
@@ -896,6 +900,19 @@ function Privacy() {
           after={<Switch variant="Primary" value={hidePresence} onChange={setHidePresence} />}
         />
       </SequenceCard>
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Warn before sending to unverified devices"
+          description="Show a warning in the composer when sending to a room with unverified devices."
+          after={
+            <Switch
+              variant="Primary"
+              value={warnOnUnverifiedDevices}
+              onChange={setWarnOnUnverifiedDevices}
+            />
+          }
+        />
+      </SequenceCard>
     </Box>
   );
 }