fix(updater): keep upgrades visible and use the checked download source

[XingYu-Zhong]

Summary

Clicking the workbench update button could cancel an available upgrade and hide the button. Download startup raced the update mirrors again; a different winner invalidated the previously checked metadata as if the user had changed channels.

Changes

• Download from the source associated with the checked update metadata and emit the downloading state immediately.
• Keep known updates visible while rechecking and show download/install failures with a dismissible error and a retryable button.
• Isolate the update button from the other workbench actions and add regression coverage for mirror changes, progress, failures, retries, and manual updates.

Tests

• 72 relevant Vitest tests passed across the updater and workbench/settings update suites.
• npm run typecheck
• npm run build
• npm run check:file-lines
• ESLint for all changed source and test files; git diff --check
• Chromium UI verification at 2x scale: button position, busy states, failure/retry, dismissible errors, and light/dark layouts.
• Final renderer tests, TypeScript check, and Electron Vite build passed after the tooltip adjustment.

The installed application was not replaced during validation.