feat: localize builtin catalog descriptions

[lilyshen0722]

Summary

• add first-party locale descriptions to builtin connector/tool catalog metadata and Installable rows
• return complete per-locale maps while keeping canonical description as the English fallback
• keep marketplace rows unmodified and project them as { en }; the API no longer negotiates Accept-Language
• use the app's i18n.resolvedLanguage ?? i18n.language via one shared frontend helper on both Connectors and Tools pages
• collapse Slack's canonical English source into the manifest and seed locale maps for Telegram, Slack, Discord, GroupMe, and GitHub
• include backend projection/source-isolation tests, frontend page/helper language coverage, and a current 1200×1000 zh-CN Connectors + Tools capture

Verification

• backend: 68 targeted catalog/route/manifest/tool/seed/installable tests passed
• backend: npm run build passed
• frontend: 109 suites, 878 tests passed
• frontend: npm run typecheck and npm run build passed; targeted lint has no errors (repository-wide lint has pre-existing unrelated errors)

Rebased onto main at 41874c46. Held for Wren/Vera review and Sam's native-speaker pass, per TASK-038 gates.