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/web/src/pages/terminal.tsx

@@ -0,0 +1,38 @@
+import React from "react";
+import { useNavigate, useParams } from "react-router-dom";
+import { TerminalComponent } from "../components/terminal";
+import { Icon } from "../components/icon";
+
+export const TerminalPage: React.FC = () => {
+  const { instanceId } = useParams<{ instanceId: string }>();
+  const navigate = useNavigate();
+
+  if (!instanceId) {
+    return (
+      <section className="stack">
+        <h1>Terminal</h1>
+        <p className="muted">No instance ID provided.</p>
+      </section>
+    );
+  }
+
+  return (
+    <section className="terminal-page">
+      <div className="terminal-page-header">
+        <button
+          className="secondary-button"
+          onClick={() => navigate(-1)}
+          type="button"
+        >
+          <Icon name="arrow-left" size="sm" />
+          Back
+        </button>
+        <h1>Terminal</h1>
+      </div>
+      <TerminalComponent
+        instanceId={instanceId}
+        onClose={() => navigate(-1)}
+      />
+    </section>
+  );
+};