fix: accept WebSocket before validation to prevent opaque 403 errors

All WebSocket endpoints now call websocket.accept() before any
validation checks. Previously, closing the connection before accepting
caused Starlette to return an opaque HTTP 403 instead of a meaningful
error message.

Changes:
- Server: Accept WebSocket first, then send JSON error + close with
  4xxx code if validation fails (expand, spec, assistant, terminal,
  main project WS)
- Server: ConnectionManager.connect() no longer calls accept() to
  avoid double-accept
- UI: Gate expand button and keyboard shortcut on hasSpec
- UI: Skip WebSocket reconnection on application error codes (4000-4999)
- UI: Update keyboard shortcuts help text

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

server/routers/terminal.py

@@ -225,9 +225,7 @@ async def terminal_websocket(websocket: WebSocket, project_name: str, terminal_i
     await websocket.accept()
 
     # Validate project name
-    try:
-        project_name = validate_project_name(project_name)
-    except Exception:
+    if not validate_project_name(project_name):
         await websocket.send_json({"type": "error", "message": "Invalid project name"})
         await websocket.close(
             code=TerminalCloseCode.INVALID_PROJECT_NAME, reason="Invalid project name"