feat(desktop): show update download progress

[AprilNEA]

Summary

• Carry electron-updater download percentage through the existing desktop updater state and system IPC bridge.
• Show a determinate, accessible progress bar with the current percentage in Settings → About while an update downloads.
• Clear progress when the updater moves to another lifecycle state and preserve the existing downloaded-update notice.

Linear: CODE-522

Verification

• pnpm check:ci
• pnpm test — 287 files passed, 2250 tests passed, 1 skipped
• pnpm -F @linkcode/desktop run build
• Launched an isolated built Electron app with Playwright, opened Settings → About, injected a 42% updater event through the real IPC channel, and observed the progress bar with aria-valuenow="42".

Checklist

• pnpm check:ci and pnpm test both pass
• I ran the affected surface and observed the change working
• Wire protocol change — N/A, this changes desktop system IPC only

[linear-code]

CODE-522