refactor(i18n): reduce fallback and l10n debt

[limityan]

Related to #959

Summary

This PR combines PR A and PR B into one i18n debt cleanup pass:

• Removes Web UI defaultValue literal fallback debt from flow-chat and Deep Review surfaces whose keys already exist in locale resources.
• Routes Web UI and Mobile Web user-visible date/number formatting through the owned i18n formatting helpers instead of per-call-site Intl / toLocale* formatting.
• Keeps i18n governance report summaries aligned with owned budget and scan definitions, including zero-count surfaces.
• Improves high-confidence zh-TW localization wording across Web UI, Mobile Web, installer, and core resources while preserving readable UTF-8 locale files.
• Adds explicit slash-command Commands copy and localizes Review Platform relative-time labels through the common namespace.
• Updates durable i18n architecture/development docs for formatting-helper ownership and call-site rules.
• Adds a test i18n helper so affected tests resolve real en-US locale resources instead of relying on inline defaultValue fallback text.

Key Metrics

Metric	Before	After	Change
Literal fallback candidates	396	149	-247
Direct locale-format candidates	24	2	-22
l10n quality candidates	1087	768	-319
Shared-term duplicate candidates	201	201	0
Mobile Web l10n candidates	19	0	-19

Risk Notes

• zh-TW wording changes are broad and may still benefit from native terminology review over time.
• Some dynamic fallback and resource-quality debt remains and continues to be tracked by the existing baselines.
• The two remaining direct locale-format candidates are core miniapp assets outside this PR's runtime cleanup scope.

Verification

• pnpm run i18n:audit
• pnpm run i18n:contract:test
• pnpm run type-check:web
• pnpm --dir src/mobile-web run type-check
• pnpm --dir src/web-ui run test:run
• pnpm run lint:web (0 errors; 2 existing startupTrace.ts warnings)
• pnpm run check:repo-hygiene