feat: crash recovery overlay for unresponsive instances

[Teamingzooper]

Summary

When an embedded webview's renderer process dies (OOM kill, V8 segfault, malicious page calling `process.crash()`, a runaway memory leak), Nexus previously just showed a frozen view with no signal to the user. This PR adds an HTML overlay over the affected instance with Reload and Dismiss buttons.

Branch base: `fix/notification-sidebar-sync` — this stacks on the open PR #6 so the teardowns + bus-forwarding patterns added there are reused here. Merge #6 first.

Mechanism

Same main → bus → IPC → renderer pattern as the notification/sidebar sync fix:

1. `ViewService` attaches `webContents.on('render-process-gone', …)` to every view at creation. Filters out clean exits. Emits a new `view:crashed` bus event with `{instanceId, reason}`.
2. `IpcService` subscribes to the bus event (alongside `instance:activated`) and forwards to the main renderer window via the new `nexus:view:crashed` IPC channel.
3. `preload.ts` exposes `window.nexus.onViewCrashed(cb)`.
4. Renderer store tracks `crashedInstances: Record<id, {reason}>` and gets two new actions: `reloadCrashedInstance(id)` and `dismissCrash(id)`.
5. New `CrashOverlay` component mounted globally in `App.tsx`. Shows only when the active instance is in the crashed map. Uses the existing `useOverlay()` hook so the underlying WebContentsView is moved offscreen while the overlay is visible — without that, the native Chromium layer would obscure our HTML.

Design choices

• No auto-reload. Crash loops are worse than a stuck panel; user controls when to retry.
• Background instances that crash are silently tracked and the overlay only shows when the user activates them. Avoids interrupting whatever else they're doing.
• Dismiss removes the entry entirely — re-activating the instance after dismissing won't re-show the overlay until it crashes again.
• Optimistic flag clearing on Reload. If the reload itself fails, the action re-flags with the new error string so the overlay reappears with a useful message.

Test plan

• `npm run typecheck` — clean
• `npm run test` — 178/178 passing
• `npm run build` — renderer + main both clean
• Manual smoke: open an instance, open its devtools, type `process.crash()`. Overlay should appear within ~200ms. Click Reload — fresh renderer spawns, overlay disappears. Try Dismiss instead — overlay hides, you can switch to another instance.
• Manual smoke: trigger a crash on a background instance (open it, switch away, then crash). No overlay shows until you activate that instance, at which point it appears.

No automated test — the same IPC integration gap as the prior PR. The change is small enough that manual smoke is the right level.

🤖 Generated with Claude Code