feat: builtin skills seed engine + Vietnamese (vi-VN) locale

[ngav1491]

Summary

Two features shipped together after a full codebase review (2026-08-23):

1. feat(skills): seed 10 builtin skills with idempotent sync engine

The Skills system previously shipped empty — no built-in skills, admin-only creation via API. This PR adds:

• Seed engine (backend/internal/application/skill/seed.go):
  • Embeds seeddata/*.md with YAML frontmatter (name/title/description/sort)
  • Validates limits: title ≤ 64 chars, description ≤ 256, body ≤ 10,000 runes
  • Idempotent on restart: missing → create; untouched record (created_by == 0 && updated_by == 0) → re-sync content on upgrade; admin-modified → never overwritten
• Wired in backend/internal/app/app.go right after skill service init
• 10 bundled skills: diagram-svg, mermaid-diagram, html-artifact, svg-icon, chart-svg, report-writing, polish-writing, translate, code-review, slide-outline — authored for DEEIX's render stack (SVG block preview, native mermaid, sandboxed HTML artifacts)
• Users see them immediately via @mention thanks to the existing scope=builtin filter
• Tests: go test ./internal/application/skill/... PASS

2. feat(i18n): add Vietnamese (vi-VN) locale

Complete third locale alongside en-US / zh-CN:

• 27/27 message files translated to frontend/i18n/messages/vi-VN/ (~4,600 strings; largest: chat.json 831, settings.json 487, admin-models.json 460). Brand names, {placeholder} tokens and ICU plural structures kept byte-identical with en-US sources — verified programmatically (key structure + placeholder + ICU skeleton checks)
• i18n/config.ts: vi-VN added to APP_LOCALES + labels; normalizeAppLocale / resolveBrowserLocale map vi*
• i18n/messages.ts: dedicated async loader
• i18n/resolve-error-message.ts: Vietnamese error catalog, fallback string, request/settings field labels, validation rules and dependency messages
• date-fns vi locale for admin-date-time-picker and admin-date-range-filter calendars
• Backend: application/auth/service.go normalizeLocale whitelist accepts vi / vi-VN

Verification

Check	Result
pnpm typecheck	✅ PASS
biome lint .	✅ PASS (588 files)
next build (production)	✅ PASS
go build ./...	✅ PASS
go test ./internal/application/auth/...	✅ PASS
go test ./internal/application/skill/...	✅ PASS

Notes / known gaps (pre-existing, not addressed here)

• Backend email templates remain zh-CN only (auth/registration.go)
• Findings from the review that motivated follow-ups (breaker classifier dead branch F1, idleTimeoutReader data race F2, xAI video poll timeout F3, dangling openai_video_generations protocol F4) are tracked separately and intentionally excluded from this PR to keep it reviewable.