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/Tooltip.tsx

@@ -38,7 +38,7 @@ export function Tooltip({ content, children, position = 'right', delay = 200 }:
       {children}
       {isVisible && (
         <div
-          className={`absolute z-50 px-3 py-1.5 text-sm font-medium text-white bg-discord-bg-floating rounded-md shadow-elevation-high whitespace-nowrap pointer-events-none ${positionClasses[position]}`}
+          className={`absolute z-50 px-3 py-1.5 text-sm font-medium text-discord-text-primary bg-discord-bg-floating rounded-md shadow-elevation-high whitespace-nowrap pointer-events-none ${positionClasses[position]}`}
         >
           {content}
         </div>