feat: persistent context-window tracker in ChatPane

Adds a floating popover above the chat input showing current
context-window usage. Modeled on Paseo's tracker.

- New hook useChatContextStats(chatId, messages) finds the latest
  assistant message in the chat with both ctx_used and ctx_max set,
  computes percent, and returns null when data unavailable.
- New component ChatContextPopover renders a small card with the
  "Context window" label, big percent, and "used / max tokens"
  subline. Hidden when stats is null.
- Color thresholds: <60% muted, 60-85 amber, >85 destructive.
- Not a portal — absolutely positioned inside a new relative
  wrapper around ChatInput in ChatPane.tsx, so it's pane-local
  (multi-pane safe).
- Live updates via the existing messages-array dependency.
- No API / schema / WS changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/components/panes/ChatPane.tsx

@@ -3,8 +3,10 @@ import { ChevronDown, Square, X } from 'lucide-react';
 import { toast } from 'sonner';
 import { api } from '@/api/client';
 import { useSessionStream } from '@/hooks/useSessionStream';
+import { useChatContextStats } from '@/hooks/useChatContextStats';
 import { MessageList } from '@/components/MessageList';
 import { ChatInput } from '@/components/ChatInput';
+import { ChatContextPopover } from '@/components/ChatContextPopover';
 import {
   DropdownMenu,
   DropdownMenuContent,
@@ -37,6 +39,7 @@ export function ChatPane({ sessionId, chatId, projectId, sessionChats }: Props)
 
   const chatMessages = stream.messages.filter((m) => m.chat_id === chatId);
   const streaming = chatMessages.some((m) => m.status === 'streaming');
+  const contextStats = useChatContextStats(chatId, chatMessages);
 
   // Auto-send next queued message when streaming completes
   useEffect(() => {
@@ -162,7 +165,10 @@ export function ChatPane({ sessionId, chatId, projectId, sessionChats }: Props)
         </div>
       )}
 
-      <ChatInput disabled={false} projectId={projectId} onSend={handleSend} onForceSend={streaming ? handleForceSend : undefined} />
+      <div className="relative">
+        <ChatContextPopover stats={contextStats} />
+        <ChatInput disabled={false} projectId={projectId} onSend={handleSend} onForceSend={streaming ? handleForceSend : undefined} />
+      </div>
     </div>
   );
 }