feat: implement web terminal for tool instances

- Add TerminalSession backend service for docker exec subprocess management
- Add TerminalManager for WebSocket session lifecycle management
- Create WebSocket endpoint at /ws/tool-instances/{id}/terminal
- Add session cookie authentication and instance ownership verification
- Install xterm.js with fit and web-links addons
- Create TerminalComponent with xterm.js integration
- Create TerminalPage with full-screen terminal view
- Add terminal route at /instances/:id/terminal
- Add terminal button to InstanceList for running instances
- Add terminal and arrow-left icons to icon registry
- Add comprehensive terminal CSS styles (dark theme, responsive)

Quality gates: typecheck ✓, lint ✓, build ✓, Python syntax ✓

apps/api/src/services/terminal_manager.py

@@ -0,0 +1,96 @@
+"""Terminal session manager for WebSocket connections."""
+
+import asyncio
+import uuid
+from typing import Any
+
+from fastapi import WebSocket
+
+from src.services.terminal_session import TerminalSession
+
+
+class TerminalManager:
+    """Manages active terminal sessions."""
+
+    def __init__(self) -> None:
+        self._sessions: dict[str, TerminalSession] = {}
+
+    async def create_session(
+        self,
+        instance_id: uuid.UUID,
+        container_id: str,
+        websocket: WebSocket,
+    ) -> TerminalSession:
+        """Create a new terminal session."""
+        session_id = str(uuid.uuid4())
+        session = TerminalSession(session_id, instance_id, container_id)
+        await session.start()
+        self._sessions[session_id] = session
+
+        # Start background tasks for I/O streaming
+        asyncio.create_task(self._read_loop(session, websocket))
+        asyncio.create_task(self._write_loop(session, websocket))
+
+        return session
+
+    async def _read_loop(self, session: TerminalSession, websocket: WebSocket) -> None:
+        """Read output from the container and send to WebSocket."""
+        try:
+            while session.is_alive() and not session._closed:
+                data = await session.read_output()
+                if data:
+                    await websocket.send_bytes(data)
+                else:
+                    await asyncio.sleep(0.01)
+        except Exception:
+            pass
+        finally:
+            await self._cleanup_session(session)
+
+    async def _write_loop(self, session: TerminalSession, websocket: WebSocket) -> None:
+        """Read input from WebSocket and send to container."""
+        try:
+            while session.is_alive() and not session._closed:
+                message = await websocket.receive()
+                if message["type"] == "websocket.receive":
+                    if "bytes" in message:
+                        await session.write_input(message["bytes"])
+                    elif "text" in message:
+                        text = message["text"]
+                        if text.startswith("{"):
+                            # Control message (JSON)
+                            import json
+                            try:
+                                ctrl = json.loads(text)
+                                if ctrl.get("type") == "resize":
+                                    await session.resize(
+                                        ctrl.get("cols", 80),
+                                        ctrl.get("rows", 24),
+                                    )
+                            except json.JSONDecodeError:
+                                pass
+                        else:
+                            await session.write_input(text.encode("utf-8"))
+                elif message["type"] == "websocket.disconnect":
+                    break
+        except Exception:
+            pass
+        finally:
+            await self._cleanup_session(session)
+
+    async def _cleanup_session(self, session: TerminalSession) -> None:
+        """Clean up a session."""
+        if session.session_id in self._sessions:
+            del self._sessions[session.session_id]
+        await session.close()
+
+    async def close_all(self) -> None:
+        """Close all active sessions."""
+        sessions = list(self._sessions.values())
+        self._sessions.clear()
+        for session in sessions:
+            await session.close()
+
+
+# Global terminal manager instance
+terminal_manager = TerminalManager()