feat(updater): add update-now action to update-available toast

[Raymond8196]

Problem

When users click "Check for updates" in Settings and a new version is available, the bottom-right toast only informs them that a new version exists ("Version X is ready to install.") with no way to act on it. Users must then hunt for the update entry point elsewhere (the sidebar update button), which is not discoverable — the toast that announced the update offers no path forward.

Solution

Add a primary action button ("Update now") to the update-available toast in notifyCheckSuccess (src/scaffold/AppUpdater/index.tsx). Clicking it calls the already-exported installAvailableAppUpdate() — the same entry point the sidebar update button uses (SidebarUpdateButton.tsx), so both paths stay consistent.

The button reuses the existing, Windows-aware install flow: download (with progress toast) → "Install and restart" confirmation modal → install + relaunch. The two-step download/confirm split is intentional (installing can terminate the app on Windows), so this change introduces no new platform behavior.

The Message component already supported an action field (used elsewhere); this change simply populates it.

Potential risks

• No new platform-specific behavior: the toast render path and the reused install entry point contain no OS branching. The actual install/relaunch mechanism remains owned by @tauri-apps/plugin-updater, unchanged by this PR.
• Behavioral consistency: the "Update now" action defaults to closeOnClick: true (Message component default), so clicking it dismisses the toast and immediately starts the download — identical to the sidebar button but with an added auto-dismiss of the announcing toast. This is the intended UX.
• i18n: the toast copy ("Update available", "Update now", "Version X is ready to install.") remains hardcoded English, matching all other AppUpdater toast strings, which are not yet i18n-keyed. Localizing these strings is out of scope for this single-concern change.

Verification

• npx eslint src/scaffold/AppUpdater/index.tsx src/scaffold/AppUpdater/index.test.ts → exit 0.
• Pre-commit hooks passed on commit (lint-staged: eslint --fix + prettier --write; scoped TypeScript type check; circular-dependency check).
• pnpm vitest run src/scaffold/AppUpdater/index.test.ts:
  • New test offers an update-now action on the available-update notice → passes (asserts the toast carries action: { label: "Update now" } with a function onClick).
  • 6 other tests fail, but these are pre-existing failures on the develop baseline (confirmed via git stash + baseline rerun): root cause is a test-infrastructure defect where index.test.ts stubs a partial window.localStorage that poisons later tests' clearSkippedUpdateVersion calls. This PR does not touch that path and the failure count is identical with/without these changes. Fixing the test infrastructure is a separate concern.
• Cross-platform: source-level analysis confirms no OS branching in the changed path; the reused install flow was already adapted for Windows. Not manually run on Windows/Linux (no behavior change introduced by this PR beyond surfacing an existing function from a new button).