feat(updater): pre-release channel opt-in toggle

[Nic-dorman]

Summary

Adds an opt-in "Pre-release builds" toggle to Settings → Advanced so internal testers running on RCs get auto-updates between RC builds, not just at stable cuts. Off by default — zero impact on the stable-channel population.

Why

tauri-plugin-updater reads its endpoint from compile-time tauri.conf.json config, and we point that at releases/latest/download/latest.json. GitHub's /releases/latest/... redirect is defined to skip releases marked prerelease=true, so RC artefacts are invisible to the in-app updater even though release.yml correctly publishes a signed latest.json to every prerelease.

Internal testers had to download every RC manually. This PR closes that gap.

How

The plugin's Builder doesn't accept runtime endpoints, so we bypass its frontend check()/Update.downloadAndInstall() entirely and own the flow ourselves via app.updater_builder().endpoints(...) (which the per-call UpdaterBuilder does support). Plugin stays registered for permissions / target detection.

Three commits, each independently buildable:

1. feat(settings): add pre-release channel toggle (UI only)

   • AppConfig.prerelease_channel: bool (default false), persisted via existing config.toml round-trip.
   • Settings → Advanced toggle bound to it.
   • Restart-hint banner that shows whenever the live value differs from the snapshot taken on first loadConfig. The snapshot represents the channel the running session resolved its endpoint URL from, so the hint only appears when a restart would actually change behaviour.

2. feat(updater): channel-aware check + install commands

   • New src-tauri/src/updater_channel.rs module with UpdaterChannelState (managed) holding a cached resolved endpoint URL and the pending Update instance.
   • check_for_update_custom reads the channel flag, resolves the endpoint (stable redirect for off; latest tag including pre-releases via the GitHub releases API for on), builds an UpdaterBuilder with that endpoint, calls .check(), stashes the Update for later install. Returns UpdateMetadata { version, body, is_prerelease }.
   • install_pending_update pulls the Update from state, runs download_and_install, re-emits Started/Progress/Finished as update-download-event with the same payload shape the JS plugin emits today (frontend handler unchanged).

3. feat(updater): switch frontend store to channel-aware Tauri commands

   • stores/updater.ts no longer imports from @tauri-apps/plugin-updater. Calls our custom commands instead. Subscribes to update-download-event for progress.
   • The markRaw(Update) workaround goes away — the Update instance now lives in Rust state, not Pinia.
   • UpdateDialog shows an amber "Pre-release" badge next to the version when meta.is_prerelease is true.

Notes

• Restart-required is the documented UX, but the implementation is more lenient. The check command re-reads the config every call and the prerelease URL is cached for the session. So flipping the toggle and clicking Check Now does pick up the new channel. Restart is still the right hint for users because the auto-check that runs on app boot uses whatever the channel was at boot.
• Does not change release.yml. The workflow's existing prerelease=true detection (*-rc.* / *-beta.* / *-alpha.*) is correct.
• Plugin remains registered — needed for permissions and target/exe detection. Its compile-time endpoint config becomes effectively dead but the plugin still expects a valid value, so leaving the existing one is fine.

Test plan

• cargo check --all-targets clean
• cargo clippy --all-targets -- -D warnings clean
• cargo fmt --check clean
• npx nuxi build clean (Nuxt + Vite)
• CI green (Linux)
• Manual: toggle off → stable check returns expected version, no badge; existing RCs invisible.
• Manual: toggle on → restart hint appears; after restart, check resolves the latest pre-release tag, dialog shows "Pre-release" amber badge.
• Manual: install end-to-end on an internal RC build (download progress, restart-on-finish).
• Manual: toggle on → toggle off → restart → next check reverts to stable.

vitest locally hits the same Windows + npm + @vue/test-utils ESM flake we've seen on every branch since #59 — pre-existing, CI on Linux is the source of truth.

Closes

Linear ticket tracking this work (internal).