feat: GitHub update checker (manual + daily startup check)

[noidee-dev]

Summary

• Adds a cross-platform update checker that compares the running version against the latest GitHub release (no native auto-updater — Electron's Squirrel updater doesn't cover Linux and needs macOS signing; this works everywhere with no new dependency).
• Settings → "Check for updates" button with status (up-to-date / version available + Download link) and a "Check on startup" toggle.
• Startup auto-check throttled to at most once per 24h (persisted via lastUpdateCheckAt); shows an informational toast when an update is available, silent on errors/offline.

How it works

• Main: pure checkForUpdate({fetchImpl, currentVersion}) → Result<UpdateInfo> hitting releases/latest; exposed via IPC channel app:checkForUpdate (handler defaults fetchImpl to globalThis.fetch).
• Settings schema gains autoCheckUpdates (default true) + lastUpdateCheckAt.
• Renderer: shouldAutoCheck 24h throttle, useUpdateCheck mutation (records the check time), Settings UI, and a render-null StartupUpdateCheck mounted inside ToastProvider.
• i18n added in all 6 locales.

Design: docs/superpowers/specs/2026-06-13-update-check-design.md · Plan: docs/superpowers/plans/2026-06-13-update-check.md

Test Plan

• npm test — 480 passing (incl. new unit/UI/integration tests for version compare, throttle, IPC handler, Settings UI, startup toast)
• npx tsc --noEmit clean · npm run lint 0 errors
• Manual: launched npm start; Settings → "Check for updates" reports "up to date" against the matching v1.2.1 release
• Reviewer: verify the "update available" path (toast + Download) e.g. by checking against a newer release tag

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