feat(desktop): add update polling and sidebar prompt

[AprilNEA]

Summary

• Check the desktop update feed at startup and every four hours while preserving electron-updater's automatic download and install-on-quit behavior.
• Replace the native downloaded dialog with a lower-left sidebar notice that offers an immediate restart and the matching GitHub Changelog.
• Keep updater state queryable through the system IPC bridge so renderer subscription timing cannot lose a downloaded update; preserve the About tab's manual check.
• Document the release-build update behavior.

Linear: CODE-512

Verification

• pnpm check:ci
• pnpm test — 284 files / 2236 tests passed; 1 skipped
• pnpm -F @linkcode/daemon build
• pnpm -F @linkcode/desktop run build
• Launched the built Electron app against the development daemon and observed the updater IPC event, lower-left notice, version, restart action, and matching Changelog URL.

Checklist

• pnpm check:ci and pnpm test both pass (plus cargo fmt / clippy / test for Rust changes)
• I ran the affected surface and observed the change working
• If a wire message changed: WIRE_PROTOCOL_VERSION is bumped — N/A, this changes desktop system IPC only
• New code and assets are my own work, or their origin and license compatibility are noted above
• Docs and comments are updated where behavior changed

[linear-code]

CODE-512

[chatgpt-codex-connector]

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