feat: add integrated terminal frontend

- Add TerminalPanel component with xterm.js, FitAddon, drag-to-resize,
  header bar (status dot, shell label, cwd, restart/minimize/close)
- Modify AppShell to accept terminal panel in vertical flex layout
- Add terminal state management in App.tsx (open/height/running state,
  Ctrl+backtick toggle, height persistence via IPC)
- Add InlineTerminalPill to composer pill row (both single/multi-agent)
- Install @xterm/xterm and @xterm/addon-fit as devDependencies
- Update ARCHITECTURE.md with frontend terminal component details
- Add Integrated Terminal feature card to marketing website

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

package.json

@@ -47,6 +47,8 @@
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "5.1.0",
+    "@xterm/addon-fit": "^0.11.0",
+    "@xterm/xterm": "^6.0.0",
     "@xyflow/react": "^12.10.1",
     "bun-types": "^1.3.11",
     "electron": "^41.0.3",