feat: MSC4260 Report User, Bug #6 mutual exclusion, TDS toast compliance

- Add ReportUserModal.tsx — category dropdown + reason input, calls
  POST /_matrix/client/v3/users/{userId}/report via mx.http.authedRequest,
  inline success/error feedback, auto-closes 1500ms after success
- Wire Report User button into UserRoomProfile.tsx between UserModeration
  and UserDeviceSessions (hidden for own profile)
- Bug #6: enforce mutual exclusion between chat backgrounds and seasonal
  themes — ChatBgGrid clears seasonal→'off' on non-'none' pick;
  SeasonalBgGrid clears chatBackground→'none' on real theme pick;
  SeasonalEffect guards against legacy persisted state at render time
- TDS: strip all hardcoded hex/rgba fallbacks from LotusToastContainer.tsx
  (var(--lt-bg-card), --lt-accent-orange, --lt-text-primary/secondary,
  --lt-accent-orange-dim/border, --lt-box-glow-orange)
- Mark Bug #6 FIXED, MSC4260 DONE, toast TDS FIXED in LOTUS_BUGS.md and
  LOTUS_TODO.md; note EventReaders + CallControls already compliant

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

src/app/components/seasonal/SeasonalEffect.tsx

@@ -797,5 +797,9 @@ export function SeasonalEffect() {
   }, [settings.seasonalThemeOverride]);
 
   if (!theme) return null;
+  // Suppress seasonal overlay when a chat background is active — both running simultaneously
+  // wastes GPU and looks cluttered. The settings UI enforces mutual exclusion on write;
+  // this guard covers any legacy state already persisted.
+  if (settings.chatBackground !== 'none') return null;
   return <SeasonalOverlay theme={theme} reduced={reduced} />;
 }