feat(updater): beta update channel + prerelease release pipeline

[Harry19081]

Summary

Adds a stable/beta update-channel system: users can opt into prerelease builds from Settings, and the release pipeline can ship betas that auto-update along their own channel.

Release pipeline (release.yaml)

• Beta releases: tag vX.Y.Z-beta.N (for the next unreleased X.Y.Z) — same workflow, marked as GitHub prerelease as before.
• Beta channel manifest: every release (stable and beta) now publishes beta.json to a rolling updater release (auto-created on first run, semver-guarded against out-of-order rebuilds). The beta channel therefore always points at the newest build of either kind, so beta users converge back onto stable once it overtakes the last beta. The stable channel keeps using the untouched releases/latest/download/latest.json alias — zero change for existing installs.
• Windows on prerelease tags: builds NSIS only (WiX/MSI rejects SemVer prerelease versions). The updater already uses the NSIS artifact; the MSI remains on stable releases.

Backend

• New check_app_update(channel) command: rebuilds the updater against the selected channel's manifest endpoint and registers the result in the webview resource table — the plugin's own Update class (download/install/close) keeps working unchanged. The channel→URL map lives in Rust so page code can never point the updater at arbitrary URLs, preserving the updater plugin's security posture.

Frontend

• general.updateChannel setting: auto | stable | beta, default auto — prerelease builds track beta, release builds track stable, so a manually installed beta DMG keeps updating along the beta channel without touching any toggle.
• Settings → General: update-channel select (Stable/Beta) next to auto-update, showing the resolved channel; picking an option pins it and re-checks immediately so a stale update from the previous channel doesn't linger.
• Switching beta → stable never downgrades; it takes effect at the next stable release (SemVer: 1.1.25-beta.N < 1.1.25).
• i18n: en / zh / zh-Hant (other locales fall back to en per existing convention).

Test plan

• cargo check + cargo clippy clean (no findings in new code)
• eslint + prettier clean on all touched files
• 32/32 vitest passing, including new suites: channel resolver, settings-schema default/validation, channel-aware check (stable/beta resolution, pinned channel, null-update, metadata wrapping, version-read failure)
• Workflow YAML validated; no circular deps
• End-to-end: push a v1.1.25-beta.1 tag and verify the updater rolling release gets beta.json, a beta install self-updates to beta.2, and a stable install stays untouched

Closes #399

[Chloe-JY]

Reviewed with fable - LGTM