(CODUX-125) Add update-status hook for the title bar

[numnx]

🧩 Task: Add update-status hook for the title bar

Automated task PR opened by Code UX for T01.

📋 Summary

Sprint: Sprint 199 · Fix Update Button · Task: T01

Fix the Electron title-bar Update button so it reflects real update availability: surface the update affordance only when a newer version exists (driven by the existing /api/system/update-status pipeline) instead of being hardcoded to always show, and communicate the available version.

🤖 Model & Provider

Provider	Codex CLI
Model	gpt-5.5

⏱️ Timing

Started	2026-07-09 03:57:16 UTC
Finished	2026-07-09 04:01:00 UTC
Duration	3m 44s

📊 Token Usage

Metric	Value
Input tokens	64,542
Cached input tokens	721,664
Output tokens	6,881
Total tokens	793,087
Invocations	1
Tool calls	38
Active time	3m 4s

Included usage estimate (subscription/local login): $0.89

✅ QA Review

⏳ QA review runs after this PR is opened. Check the task card in the dashboard for the outcome once it lands.

📝 Full Task Prompt

## Objective
Add a small, testable Preact hook that loads Code UX update availability once on mount from the existing `/api/system/update-status` endpoint and exposes whether an update is available plus the latest version, so the title bar can render the Update button conditionally.

## Scope
- Create `dashboard/src/v2/hooks/use-update-status.ts`
- Create `tests/dashboard/hooks/use-update-status.test.tsx`
- Reuse the existing client helper `fetchUpdateStatus` and `UpdateStatus` type from `dashboard/src/v2/lib/system-api.ts` (do not redefine the contract)
- Follow the existing hook style in `dashboard/src/v2/hooks/` (e.g. `use-notifications.ts`, `use-project-git-status.ts`)

## Implementation Requirements
1. Export `useUpdateStatus()` returning a typed object, e.g. `{ status: UpdateStatus | null; updateAvailable: boolean; latestVersion: string | null }`, initialized to a not-yet-loaded state.
2. On mount, call `fetchUpdateStatus()` inside a `useEffect`; store the resolved `UpdateStatus` in state via `useState`. Use a `cancelled` guard in the effect cleanup so state is not set after unmount.
3. Derive `updateAvailable` strictly from `status?.updateAvailable === true` and `latestVersion` from `status?.latestVersion ?? null`; treat a fetch rejection as no update available (catch the error, leave `updateAvailable` false, do not throw).
4. Keep the hook side-effect free beyond the single fetch; do not add polling, intervals, or new API routes.
5. In the test file (add the `@vitest-environment jsdom` pragma at the top), mock `../../dashboard/src/v2/lib/system-api` (or the correct relative path) with `vi.mock`, render the hook via `@testing-library/preact` `renderHook`, and assert: (a) update-available payload yields `updateAvailable: true` with the latest version, (b) up-to-date payload yields `updateAvailable: false`, (c) a rejected fetch yields `updateAvailable: false` and does not throw.

## Constraints
- Do not introduce a new HTTP route, new client protocol shape, or a new copy of `UpdateStatus`; reuse `system-api.ts`.
- Do not change `fetchUpdateStatus` behavior or the server route.
- The hook must be safe to import in the Electron-rendered dashboard bundle (no Node-only APIs).

## Verification
- `pnpm test tests/dashboard/hooks/use-update-status.test.tsx` passes.
- `pnpm run typecheck:dashboard` passes.
- Confirm the three test cases (available / up-to-date / fetch error) behave as specified.

🌿 Branch Info

Base: feature/CODUX-199-fix-update-button
Head: task/feature-codux-199-fix-update-button-t01-codex-mrcz6y7o

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🤖 Generated by Code UX