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

[me2seeks]

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 feat(i18n): add Traditional Chinese locale #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 feat(i18n): add Traditional Chinese locale #3853 review)
• Explicit ko for non-catalog native fallbacks: Agent Graph, WorkHub, MCP, readiness, notifications (same list as feat(i18n): add Traditional Chinese locale #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