fix: complete UI localization coverage

[thynomex]

Result / 用户结果

Completes English and Simplified Chinese UI localization across the GPUI and legacy shells, including SSH, file dialogs, workspace actions, backup/sync notices, update notices, and VCS relative-time labels. Missing translation IDs and compile-time localization blockers are fixed, and the legacy update warning retains its release-page URL.

Design / 设计边界

• Responsibility and affected modules: translation macro/runtime, generated catalogs, shell UI call sites, SSH/file-dialog/update/VCS message paths.
• Why this belongs here; interfaces that remain unchanged: existing Message, UiLanguage, tr, and tr_args APIs remain authoritative; new shell helpers keep large modules within their existing architecture budgets.
• Dependency, data-format, threading, or lifetime changes (ADR if applicable): no persisted format, dependency, or threading changes.
• Compatibility and migration/fallback behavior: English remains the fallback; catalog lookups preserve raw-key fallback behavior for unknown IDs; native dialog filters are translated at presentation time.

Evidence / 验证依据

• cargo check -p nebula using the pinned Rust 1.97.1 toolchain: passed (existing warnings only).
• Focused i18n tests: 8 passed, 1 ignored manual benchmark.
• python scripts/check_architecture.py --base HEAD: passed; display/mod.rs remains within its ratcheted allowance.
• git diff --check: passed.
• Regression coverage includes locale negotiation, catalog synchronization, placeholder formatting, typed/compatibility lookup agreement, and singular/plural VCS relative-time boundaries.
• Full product/UI, platform packaging, and DPI screenshot checks were not run in this environment.

Required Review / 必须确认

• I followed CONTRIBUTING.md, docs/architecture.md, and docs/project-constraints.md.
• I split responsibilities, not arbitrary line ranges; no duplicate behavior authority was added.
• python scripts/check_architecture.py --base HEAD passes; budgets were not inflated to fit the change.
• Tests cover success and failure; platform/feature coverage limitations are stated.
• New messages use typed i18n IDs and matching placeholders; untranslated content has an explicit fallback.
• Governance changes include a counterexample, corrected contract, tests, and a maintainer-reviewed decision. (Not applicable.)