feat(i18n-ko): native surfaces and E2E fixtures

## Problem

Native surfaces and E2E fixtures have no `ko` coverage. Startup dialogs, quit/boot messages, and fixtures still assume `zh`/`en` only, so `auto` + `ko` may regress to `en` or show wrong fallback.

## Desired outcome

- Carry resolved `ko` through startup/quit/boot dialogs and persisted `DesktopLocalePreference` (as #3853 did for `zh-TW`)
- Preserve canonical `ko` E2E fixture flags in `apps/desktop/e2e/fixtures.ts` / `e2e-fixture.ts` (no lowercasing bug like `zh-CN`/`zh-TW` fix in #3853 review)
- Explicit `ko` for non-catalog native fallbacks: Agent Graph, WorkHub, MCP, readiness, notifications (same list as #3853)
- Add `ko` to `zh-tw-locale-screenshots.yml` style workflow or extend existing one for Korean screenshots (macos-15, real Settings screenshots as artifact)

## Alternatives or workarounds

Leave native/E2E in `en`/`zh` and accept untested `ko` paths.

## Acceptance criteria

- [ ] `auto` + `ko-KR` system → `ko` before settings are loaded (startup dialogs correct)
- [ ] E2E fixture `ko` flag preserved (no `toLowerCase` → `null` bug)
- [ ] `apps/desktop/e2e/zh-tw-locale-screenshots.spec.ts` style spec added for `ko` or extended
- [ ] Desktop locale-focused regression suites + CLI locale suites green with `ko`

Parent: #3974