Add extensible language packs and Simplified Chinese localization

[DTW7607]

Summary

• Add a language-pack registry that discovers bundled BCP-47 lproj resources without a hard-coded Swift language list.
• Add System, English, and Simplified Chinese language choices with immediate in-app updates and English fallback for unsupported system languages.
• Localize the menu bar, onboarding, settings, sheets, file panels, accessibility labels, errors, model display names, plural text, and hover captions.
• Keep the language choice global and independent from profiles, imported settings, per-app overrides, and existing Codable/raw-value formats.
• Refresh stationary hover captions after a language change without replaying haptic feedback.
• Add strict resource, BCP-47, printf-placeholder, plural, runtime, profile, shortcut, and hover regression validation.
• Add an opt-in stable local Debug signing workflow that detects an installed Apple Development identity, refuses ad-hoc fallback, and stores no personal signing identifiers in the repository.

Extensibility

A new language only requires a matching lproj resource directory and Xcode region metadata. English remains the complete key baseline and per-key fallback. See docs/localization.md for the package contract and contribution steps.

Compatibility

• Existing profile JSON and KeyCombo data remain compatible.
• Enum raw values, app version, update feed, Sparkle key, and shared release signing configuration are unchanged.
• The local signing script overrides signing only for its own build and preserves the upstream release team settings.
• User names, app names, file names, URLs, brands, and external control titles remain verbatim.

Validation

• bash scripts/test_localization.sh
• bash scripts/build_local_signed.sh
• Certificate-signed app passes strict deep codesign verification and has a certificate-based designated requirement rather than an ad-hoc code-hash identity
• 379 ordinary keys and 1 plural key in each published language pack
• 29 matcher cases and 12 identifier-normalization checks
• 24 validator self-test assertions
• Production Localizer runtime checks against source and built app resources
• Profile, shortcut, live-language, and hover-caption regression checks
• Unsigned validation build and certificate-signed local Debug build with Xcode 26.6
• Manual Simplified Chinese UI review completed on macOS, including window startup, text wrapping, terminology, and haptic-row alignment

Sparkle-owned and other system-owned windows may continue to follow the macOS language. Public distribution still requires the upstream Developer ID and notarization process.