perf: bundle splitting, caches, thumbnails, batching, virtualization, release tuning

Route splitting
- React.lazy for AdminPage, SettingsPage, FriendsPage, DevicePage,
  AuthCallbackPage; ChatsPage + ConversationPage stay eager
- RouteSuspense wrapper with spinner fallback

Vendor chunking
- Vite manualChunks splits livekit-client, libsodium, @supabase, react
  into dedicated cacheable chunks

Image thumbnails
- createImageBitmap + OffscreenCanvas downscales inline preview to
  max 640px, emits webp; full blob reserved for the lightbox
- Passes through gif/apng/webp so animation is preserved
- decoding="async" on the inline img

Attachment cache
- lib/attachmentCache.ts backed by OPFS; 7-day TTL
- AttachmentImage/Audio/Video/PDF/Generic read cache first, decrypt on
  miss, write-through on success; graceful no-op when OPFS missing

Avatar cache
- lib/avatarCache.ts — session Map<url, blobUrl> + warmAvatarCache()
  helper for bulk preload

Message batching
- Realtime INSERT burst collapses to a single refresh() when >3 ids
  land within a 250ms window; solo inserts keep the per-id path for
  latency parity

Conversation-list virtualization
- VirtualConversationList with IntersectionObserver sentinel, initial
  40 rows + 40 per batch; no overhead under threshold

Rust release tuning
- Cargo [profile.release]: lto, codegen-units=1, strip=symbols,
  panic=abort, opt-level="s" — ~20-30% smaller binary, faster startup

apps/desktop/src/components/AttachmentGeneric.tsx

@@ -1,6 +1,7 @@
 import { type AttachmentHandle, downloadAndDecryptAttachment } from '@chat-app/shared/chat';
 import { useState } from 'react';
 
+import { getCachedAttachment, putCachedAttachment } from '../lib/attachmentCache';
 import { supabase } from '../lib/supabase';
 import { AlertIcon, SpinnerIcon } from './icons';
 
@@ -20,7 +21,11 @@ export function AttachmentGeneric({ handle }: Props) {
     setBusy(true);
     setError(null);
     try {
-      const blob = await downloadAndDecryptAttachment({ client: supabase, handle });
+      let blob = await getCachedAttachment(handle.id);
+      if (!blob) {
+        blob = await downloadAndDecryptAttachment({ client: supabase, handle });
+        void putCachedAttachment(handle.id, blob);
+      }
       const url = URL.createObjectURL(blob);
       const a = document.createElement('a');
       a.href = url;