Problem

If a webview's renderer process crashes (OOM, GPU issue), the pane goes blank with no recovery path. Electron emits `render-process-gone` but we don't handle it.

Proposed approach

• Listen for `render-process-gone` on each webview.
• Show the existing error overlay (from v3) with a "Reload pane" button.
• Optionally log the crash reason via console.

Files to touch

• `src/renderer.js` — webview event listeners

Acceptance criteria

• Killing a webview's renderer via DevTools surfaces the error overlay.
• Clicking Reload restores the pane.