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/MessageInput.tsx

@@ -1,6 +1,6 @@
 import React, { useState, useRef, useCallback } from 'react';
 import { useChatStore } from '../../stores/chatStore';
-import { useUIStore } from '../../stores/uiStore';
+import { isDmChannel } from '../../stores/serverStore';
 import { wsSend } from '../../hooks/useWebSocket';
 import { api } from '../../api/client';
 
@@ -22,7 +22,7 @@ export function MessageInput({ channelId, channelName }: MessageInputProps) {
 
   const handleTyping = useCallback(() => {
     if (typingTimeoutRef.current) return;
-    const isDm = useUIStore.getState().showDms;
+    const isDm = isDmChannel(channelId);
     if (isDm) {
       wsSend({ type: 'dm_typing_start', dmChannelId: channelId });
     } else {