fix: cancel flash-message timers; make store init() idempotent

[Teamingzooper]

Summary

Two latent bugs spotted during the audit pass at the end of the recent feature batch. Both are real but minor — no user-visible breakage today, but they cause React warnings or accumulating listeners in edge cases.

Branch base: `feat/command-palette-actions` — ninth and final in the recent stack.

Bug 1 — Flash-message timers leak on unmount

`SettingsPanel` and `ThemeEditor` both use the pattern:

```ts
setMsg('Saved'); setTimeout(() => setMsg(null), 4000);
```

without ever cancelling the timer. If the user closes the settings modal within those 4 seconds, React 18 logs "can't perform a React state update on an unmounted component" and the pending setState runs on a stale component instance.

Fix: new `useFlash` hook in `src/renderer/hooks/useFlash.ts` that:

• Stores the timer id in a `useRef`.
• Cancels the timer on unmount.
• Cancels any previous timer when called again (so two flashes in quick succession don't fight).

Both components now use it; all 8 raw `setTimeout(() => setX(null), ...)` sites are gone.

Bug 2 — Store `init()` accumulates listeners across retries

`store.init()` subscribes to five `window.nexus.onX(...)` events (unread, instance-activated, view-crashed, instance-hibernated, instance-woken). The Retry button on the error screen calls `init()` again, which would have added a second set of listeners — every event would then fire its state-update closure twice (or N times after N retries).

Fix: module-scoped `initDisposers` array. `init()` calls `disposeInitSubscriptions()` at the top before creating new ones. The `onX` IPC helpers already return a remove-listener disposer, so we just collect and replay them.

Test plan

• `npm run typecheck` — clean
• `npm run test` — 178/178 passing
• `npm run build` — renderer + main both clean
• Manual smoke: open Settings → click "Send test notification" → immediately close the modal (within 4s). DevTools console should be quiet (no "setState on unmounted component" warning).
• Manual smoke: trigger an init error (e.g. kill the main process briefly), see the error screen, click Retry. Each retry should leave exactly one listener per event, not N.

🤖 Generated with Claude Code