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/ui/Avatar.tsx

@@ -66,7 +66,7 @@ export function Avatar({ src, name, size = 40, status, className = '', onClick,
       </div>
       {status && (
         <div
-          className={`absolute -bottom-0.5 -right-0.5 rounded-full border-[3px] border-[#2b2d31] ${statusColors[status] ?? 'bg-discord-text-muted'}`}
+          className={`absolute -bottom-0.5 -right-0.5 rounded-full border-[3px] border-discord-bg-secondary ${statusColors[status] ?? 'bg-discord-text-muted'}`}
           style={{
             width: size * 0.35,
             height: size * 0.35,