fix: accept WebSocket before validation to prevent opaque 403 errors

All 5 WebSocket endpoints (expand, spec, assistant, terminal, project)
were closing the connection before calling accept() when validation
failed. Starlette converts pre-accept close into an HTTP 403, giving
clients no meaningful error information.

Server changes:
- Move websocket.accept() before all validation checks in every WS handler
- Send JSON error message before closing so clients get actionable errors
- Fix validate_project_name usage (raises HTTPException, not returns bool)
- ConnectionManager.connect() no longer calls accept() (caller's job)

Client changes:
- All 3 WS hooks (useWebSocket, useExpandChat, useSpecChat) skip
  reconnection on 4xxx close codes (application errors won't self-resolve)
- Gate expand button, keyboard shortcut, and modal on hasSpec
- Add hasSpec to useEffect dependency array to prevent stale closure
- Update keyboard shortcuts help text for E key context

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/routers/assistant_chat.py

@@ -217,20 +217,26 @@ async def assistant_chat_websocket(websocket: WebSocket, project_name: str):
     - {"type": "error", "content": "..."} - Error message
     - {"type": "pong"} - Keep-alive pong
     """
-    if not validate_project_name(project_name):
+    # Always accept WebSocket first to avoid opaque 403 errors
+    await websocket.accept()
+
+    try:
+        project_name = validate_project_name(project_name)
+    except HTTPException:
+        await websocket.send_json({"type": "error", "content": "Invalid project name"})
         await websocket.close(code=4000, reason="Invalid project name")
         return
 
     project_dir = _get_project_path(project_name)
     if not project_dir:
+        await websocket.send_json({"type": "error", "content": "Project not found in registry"})
         await websocket.close(code=4004, reason="Project not found in registry")
         return
 
     if not project_dir.exists():
+        await websocket.send_json({"type": "error", "content": "Project directory not found"})
         await websocket.close(code=4004, reason="Project directory not found")
         return
-
-    await websocket.accept()
     logger.info(f"Assistant WebSocket connected for project: {project_name}")
 
     session: Optional[AssistantChatSession] = None