fix(i18n): route desktop fallback copy through catalog

[DrMaks22]

Summary

• route the Desktop intro fallback copy through the existing i18n catalog instead of hardcoded English in intro.tsx
• move the Mixture of Agents settings copy and stale auxiliary-model warning into settings.model
• add typed English source strings plus zh catalog entries for the newly exposed keys

Why

This follow-up came from the RU Desktop localization QA in #46634. The remaining English screenshots were not only missing Russian copy: some strings were still hardcoded in components, so they need a small shared catalog/component change before any locale can translate them cleanly.

Keeping this separate lets #46634 stay focused on adding the Russian locale, while this PR handles the cross-locale Desktop i18n surface.

Validation

• git diff --check
• npm install --workspace apps/desktop
• npm run typecheck --workspace apps/desktop
• npm run test:ui --workspace apps/desktop -- src/i18n/languages.test.ts src/i18n/runtime.test.ts src/i18n/context.test.tsx src/components/language-switcher.test.tsx → 4 files / 22 tests passed

Related: #46634

[teknium1]

Thanks for splitting the shared Desktop i18n surface out of the Russian-locale work.

Problems

• This changes catalog selection in apps/desktop/src/components/chat/intro.tsx and localized warning/MoA rendering in apps/desktop/src/app/settings/model-settings.tsx, but adds no regression tests. Existing intro coverage only checks layout clearance at apps/desktop/src/components/assistant-ui/thread/streaming.test.tsx:426-430; existing warning tests assert the English path at apps/desktop/src/app/settings/model-settings.test.tsx:151-180.

Suggested changes

• Add a focused Intro i18n test and a ModelSettings test under I18nProvider with the zh locale, covering the catalog-backed fallback and stale-aux/MoA strings.

Automated hermes-sweeper review.

[DrMaks22]

Thanks for the focused review. I added the requested regression coverage in commit 88704f8c2:

• apps/desktop/src/components/chat/intro.test.tsx covers the Intro catalog fallback path under I18nProvider with the zh locale.
• apps/desktop/src/app/settings/model-settings.test.tsx now wraps ModelSettings in a real QueryClientProvider harness and adds a zh I18nProvider case covering the localized stale-aux warning plus MoA preset/reference/aggregator strings.

Validation after the update:

• git diff --check
• npm run typecheck --workspace apps/desktop
• npm run test:ui --workspace apps/desktop -- src/i18n/languages.test.ts src/i18n/runtime.test.ts src/i18n/context.test.tsx src/components/language-switcher.test.tsx src/components/chat/intro.test.tsx src/app/settings/model-settings.test.tsx -> 6 files / 31 tests passed

[DrMaks22]

Updated this PR onto current main (head 415cea968) and resolved the i18n catalog conflict by preserving the newer upstream fallback-model keys while keeping the MoA / Intro / stale-aux strings catalog-backed.\n\nValidation on the refreshed branch:\n- git diff --check\n- npm run typecheck --workspace apps/desktop\n- npm run test:ui --workspace apps/desktop -- src/i18n/languages.test.ts src/i18n/runtime.test.ts src/i18n/context.test.tsx src/components/language-switcher.test.tsx src/components/chat/intro.test.tsx src/app/settings/model-settings.test.tsx -> 6 files / 31 tests passed\n\nThe PR remains scoped to the shared Desktop i18n surface and does not include the Russian locale itself.