fix: address confirmed bugs from LOTUS_BUGS.md audit

- useFileDrop: reset drag overlay when mouse leaves browser window
  (relatedTarget === null signals viewport exit, counter was getting stuck)
- useDeviceVerificationStatus: add member count to useMemo deps so new
  room members' devices get checked, not just initial joined members
- index.css: define --bg-surface-variant used by VoiceMessageRecorder,
  MessageSearch, SearchFilters, UserRoomProfile (was falling back to transparent)
- syntaxHighlight: fix Python inline comments — # after space/tab was
  treated as plain text; only start-of-line was recognised
- usePresenceUpdater: replace internal baseUrl cast with mx.getHomeserverUrl()
- useLocalMessageSearch: scan all linked timelines via getUnfilteredTimelineSet()
  not just the live window, so scrolled-back history is included in E2EE search
- RoomViewHeader: show search button in encrypted rooms — local search is
  implemented and handles them; the guard was a holdover from before it existed
- recent-emoji: return emojis in recency order (array is already unshifted on
  use) instead of sorting by total usage count

Skipped: media gallery memory leak (needs virtualization refactor),
bookmark race condition (needs queue/lock), Night Light portal coverage
(position:fixed already covers full viewport — not a real bug).

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

src/index.css

@@ -12,6 +12,7 @@
   /* semantic surface vars used by poll, location, upload card, gif picker */
   --bg-surface: #ffffff;
   --bg-surface-low: rgba(0, 0, 0, 0.04);
+  --bg-surface-variant: rgba(0, 0, 0, 0.07);
   --bg-surface-active: rgba(0, 0, 0, 0.1);
   --bg-surface-border: rgba(0, 0, 0, 0.14);
   --text-primary: #1a1a1a;
@@ -37,6 +38,7 @@
   /* semantic surface vars — dark overrides */
   --bg-surface: #25272e;
   --bg-surface-low: rgba(255, 255, 255, 0.05);
+  --bg-surface-variant: rgba(255, 255, 255, 0.08);
   --bg-surface-active: rgba(255, 255, 255, 0.1);
   --bg-surface-border: rgba(255, 255, 255, 0.12);
   --text-primary: #e0e5ed;