refactor(i18n): harden locale debt governance

[limityan]

Summary

This PR consolidates the next i18n governance cleanup pass into one fork PR:

• adds machine-readable audit report sections for Web UI literal defaultValue fallbacks and direct locale-formatting calls;
• introduces a no-growth baseline for direct Intl.* / toLocale* usage outside shared i18n formatting helpers;
• removes redundant literal fallback copy from selected Web UI call sites now covered by static key auditing;
• moves the mobile-web search tool label to shared.tools.search and removes local copies;
• updates selected zh-TW installer/mobile strings where Simplified Chinese copy was still used;
• keeps i18n guidance concise in AGENTS.md, CONTRIBUTING.md, and durable i18n docs.

Related: #959

Key Metrics

Metric	Before	After	Delta
Confirmed unused keys	0	0	0
Dynamic key candidates	194	193	-1
Shared-term duplicate candidates	206	201	-5
L10n quality candidates	1093	1087	-6
Literal defaultValue fallbacks	457	396	-61
Direct locale-format calls	32	24	-8

Notes For Reviewers

• shared is still explicit, not a global fallback namespace.
• Removed Web UI defaultValue strings are for static keys that i18n:audit verifies exist.
• Direct locale formatting is now audited as debt with a no-growth baseline. The audit catches new Intl.*, bare Intl.*, and toLocale* call sites outside approved i18n formatting helpers.
• The remaining direct locale-format calls are intentionally left as tracked debt for future focused cleanup.
• CI follow-up fixed Web UI tests that were still relying on literal fallback behavior or incomplete react-i18next mocks.

Risk / Behavior Changes

• Number and timestamp formatting in the touched Web UI paths now follows the active app locale via i18nService / useI18n instead of browser/default Intl behavior.
• Mobile-web search tool labeling now reads the canonical shared tool label.
• zh-TW installer/mobile wording changes are visible copy improvements; they do not change resource shape or placeholders.
• lint:web exits 0 but still reports two pre-existing warnings in src/web-ui/src/shared/utils/startupTrace.ts about unused eslint-disable directives.
• Web and mobile builds pass locally with existing Vite chunk-size/dynamic-import warnings.

Verification

• GitHub Actions CI passed on a3c966bf: Frontend Build, Rust Build Check (ubuntu-latest), Rust Build Check (macos-15), Rust Build Check (windows-latest)
• pnpm run i18n:contract:test
• pnpm run i18n:audit
• pnpm run i18n:audit -- --report-json <temp>
• pnpm --dir src/web-ui run test:run (157 files / 802 tests)
• pnpm run type-check:web
• pnpm run lint:web (exit 0; two existing warnings noted above)
• pnpm --dir src/mobile-web run type-check
• pnpm run build:web
• pnpm run build:mobile-web
• pnpm run check:repo-hygiene
• pnpm run check:github-config
• git diff --check gcwing/main..HEAD