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/Modal.tsx

@@ -27,10 +27,10 @@ export function Modal({ isOpen, onClose, title, children, maxWidth = 'max-w-md'
   return (
     <div className="fixed inset-0 z-50 flex items-center justify-center animate-fade-in">
       <div
-        className="absolute inset-0 bg-black/70"
+        className="absolute inset-0 bg-discord-bg-overlay"
         onClick={onClose}
       />
-      <div className={`relative ${maxWidth} w-full mx-4 bg-discord-bg-primary rounded-lg shadow-xl animate-slide-up`}>
+      <div className={`relative ${maxWidth} w-full mx-4 bg-discord-bg-surface rounded-lg shadow-xl animate-slide-up`}>
         {title && (
           <div className="flex items-center justify-between px-4 pt-4">
             <h2 className="text-xl font-bold text-discord-text-primary">{title}</h2>