Add Simplified Chinese (zh-CN) UI localization

[peterwongai2026-beep]

Implements #64: Simplified Chinese (zh-CN) UI localization. Based on release/0.7.0.

Approach

• No new runtime dependency. Small in-repo engine: catalogue lookup, per-message English fallback, {name} interpolation.
• English is the source of truth; its keys form the MessageKey type, so an unknown key fails typecheck. zh-CN is Partial, so translations may lag English.
• System locale by default, with a manual "Interface language" override in Settings persisted in userData (ui-settings.json). zh, zh-CN, zh-SG, zh-Hans* resolve to Simplified; zh-TW/zh-HK/others fall back to English.
• Main process broadcasts language changes to every window (recorder, controls, library, terminal each have their own React root); window/tray titles update too.
• UI language is independent of the narration transcription language and of generated Skill/Automation language (unchanged).

Scope

Migrated renderer surfaces: Recorder, RecordingControls, RecordingPrivacyWarning, WhatsRecorded, SensitiveReview, SkillReviewModal, Terminal, Library, plan-edit, format/skill-placement helpers. English output is unchanged.

Validation

• npm run typecheck, npm test, npm run build pass.
• New tests: locale resolution, fallback/interpolation, zh-CN key coverage, UI-language persistence.
• Manually checked the zh-CN UI in a dev build (macOS arm64). Could not produce a packaged build locally (compliance downloads returned 403).

Not included

• Localizing generated Skill output language and Whisper language names.
• Traditional Chinese catalogue.

Note: overlaps with draft #72 (based on release/0.6.0); happy to reconcile with maintainers' preference.

Refs #64

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