feat: add backend status indicator and restart controls

[randroid88]

Summary

• Live backend status indicator (green/red/yellow dot) in the toolbar
• Adaptive health polling: 1s retry when offline, 5s heartbeat when online
• Restart/start controls in both Electron (IPC) and browser mode (Vite middleware)
• Backend status section in Settings panel with status display + action button

Before

No visibility into whether the backend is running. If the backend crashes or hasn't started, the user gets cryptic fetch errors with no guidance.

After

A status dot in the toolbar provides instant feedback:

• Green: Backend is online and healthy
• Red: Backend is offline — click "Start Backend" in Settings
• Yellow (pulsing): Connecting/checking status

The Settings panel shows a dedicated Backend section with the status, backend URL, and a restart/start button that works in both Electron and browser mode.

Implementation details

• BackendStatusDot: Color-coded dot component with tooltip
• utils/env.ts: Shared IS_ELECTRON constant, startBackend() helper, status labels — replaces scattered !!window.electronAPI checks
• editorStore: New backendStatus + setBackendStatus state
• SettingsPanel: Backend status section with restart button
• App.tsx: Health-check polling useEffect with adaptive interval
• electron/main.js: backend:restart IPC handler (stops, waits 600ms for port release, restarts)
• vite.config.ts: backendLauncher Vite plugin — POST /api/start-backend spawns the Python backend in browser dev mode, with 2s crash cooldown

Files changed

File	Change
frontend/src/types/project.ts	Add BackendStatus type
frontend/src/utils/env.ts	New: IS_ELECTRON, startBackend(), BACKEND_STATUS_LABEL
frontend/src/components/BackendStatusDot.tsx	New: status dot component
frontend/src/store/editorStore.ts	Add backendStatus, setBackendStatus
frontend/src/store/aiStore.ts	Use IS_ELECTRON from env.ts
frontend/src/components/SettingsPanel.tsx	Add backend status section
frontend/src/App.tsx	Health polling, import BackendStatusDot
frontend/src/vite-env.d.ts	Add restartBackend to ElectronAPI
frontend/vite.config.ts	Add backendLauncher plugin
electron/main.js	Add backend:restart IPC handler
electron/preload.js	Expose restartBackend

Test plan

• Start the app without the backend running — dot should be red, Settings shows "Offline"
• Start the backend — dot turns green within 1-2 seconds
• Kill the backend — dot turns red within 1-2 seconds
• Click "Restart Backend" in Electron — backend restarts, dot goes yellow then green
• In browser mode (npm run dev:frontend), click "Start Backend" — backend spawns via Vite middleware
• Verify crash cooldown: kill backend repeatedly — should not rapid-fire respawns