feat: unread indicators + DM bug fixes + data-driven isDmChannel

- Fix stale message cache: add force param to loadMessages, clearAllMessages action
- Fix reload race condition: URL-based isDmChannel fallback before WS ready
- Add read_states DB table for persistent unread tracking
- Add channel_ack WS event (client→server→echo) with BigInt comparison
- Wire up unread state in chatStore (readStates, unreadChannels, ackChannel)
- Auto-ack channels on MessageList view (200ms debounced)
- Unread pill indicators on server icons in ServerSidebar
- Bold text + white dot on unread channels/DMs in ChannelSidebar
- Replace all showDms reads with data-driven isDmChannel() across 8 files
- Design system, UI polish, and component fixes from previous sessions

packages/web/src/components/chat/ImagePreview.tsx

@@ -10,7 +10,7 @@ export function ImagePreview() {
 
   return (
     <div
-      className="fixed inset-0 z-[60] flex items-center justify-center bg-black/80 animate-fade-in cursor-pointer"
+      className="fixed inset-0 z-[60] flex items-center justify-center bg-discord-bg-overlay animate-fade-in cursor-pointer"
       onClick={closeImagePreview}
     >
       <button
@@ -24,7 +24,7 @@ export function ImagePreview() {
       <img
         src={imageUrl}
         alt="Preview"
-        className="max-w-[90vw] max-h-[90vh] object-contain rounded shadow-2xl"
+        className="max-w-[90vw] max-h-[90vh] object-contain rounded shadow-elevation-high"
         onClick={(e) => e.stopPropagation()}
       />
     </div>