Add localization infrastructure and Simplified Chinese translation

[serenNan]

Closes #629

Summary

Implements #629: adds Apple localization infrastructure and ships Simplified Chinese (zh-Hans) as the first translation.

• Infrastructure — enables SWIFT_EMIT_LOC_STRINGS to extract LocalizedStringKey literals at build time, declares en + zh-Hans in CFBundleLocalizations, and adds Localizable.xcstrings (English source of truth) wired into the Tuist project. Existing SwiftUI Text/Label call sites keep working unchanged.
• Translations — zh-Hans for 450 user-facing strings across settings, sidebar, toolbar, onboarding, notifications, and the command palette. Anything untranslated falls back to English.
• In-app language switcher — a General-settings picker (Follow System / English / 简体中文) that persists the choice and offers a one-tap relaunch to apply it, layered on top of the system per-app language override.

The app follows the system language by default; the in-app picker is an optional convenience.

Commits are split so each is easy to review in isolation:

1. Add Simplified Chinese localization infrastructure — catalog + project wiring only.
2. Add in-app language switcher in General settings — the picker, AppLanguage model, relaunch, and unit tests.
3. Translate UI to Simplified Chinese — the zh-Hans catalog entries.

Type of change

• Bug fix (the linked issue is a bug report)
• Feature (the linked issue is a feature request marked ready)
• Documentation
• Other (please describe)

Note: #629 is a feature request that a maintainer still needs to mark ready. The policy check will flag this until then — happy to wait for approval or adjust scope.

How was this tested?

• make check passes (format + lint) — ran swift-format lint --strict and swiftlint over the changed files; clean.
• make test passes — ran the new AppLanguage / settings-language unit tests (8 tests, all green) via xcodebuild test; leaving the full suite to CI.
• I built and ran the app to confirm the change works — verified the UI renders in Simplified Chinese and the in-app switcher applies the language after relaunch.

AI tool disclosure (optional)

• Model(s): Claude Opus 4.8
• Harness / tools: Claude Code

Checklist

• This pull request is linked to an issue with Closes # above.
• For a feature, the linked issue is labeled ready. — pending maintainer approval of Localization support with Simplified Chinese as the first translation #629.
• I am the author of this work and accountable for it; no commit is authored or co-authored by an AI agent.
• I have read the Contributing guide and the Code of Conduct.

[github-actions]

Thanks for the pull request. It doesn't meet the contribution policy yet, so I've labeled it invalid. Please address the following, then push an update:

• Linked issue Localization support with Simplified Chinese as the first translation #629 is not marked ready yet. Bugs are ready on filing; features are marked ready by a maintainer once approved.

Nothing needs to be redone. A pull request left invalid is closed automatically after a few days of inactivity. See CONTRIBUTING.md for the full flow.

[serenNan]

Closing for now: I opened this before #629 is marked ready. The localization work is complete and the pr-629 branch is ready — I'll reopen this as soon as a maintainer approves #629 with the ready label. Sorry for the premature PR.

[sbertix]

Added ready label to the issue.
I'll review it later once it's reopened, but please remove the language switch from settings. If we support localization, we must do it the "native way".

[serenNan]

Superseded by #646 (this one couldn't be reopened after a force-push). The language switcher has been removed there per the review feedback.