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.

[lilyshen0722]

Cross-reference, not a review: this PR is the home for the last item of TASK-179 (follow-ups carried out of #1981, the Connectors/Tools i18n migration).

That item is the server-English builtin catalogue description — the text entry.description renders at V2ConnectorTools.tsx:572 (not-yet row) and :671 (draft aside), which arrives from the API and so cannot be reached by either locale catalog. lily-shen's instruction for it was to route this PR rather than rebuild it, and #1982 carries no server-side change because of that.

Two things a reviewer will want to know, measured today rather than assumed:

• this branch is 11 days old and its base has moved a long way (main is 827b47e); the Connectors/Tools components it edits have changed twice since (fix(v2): the connectors row's hover, name gutter, and line 3 (TASK-162) #1978 TASK-162, fix(v2): route the Connectors and Tools copy through i18n (TASK-164) #1981 TASK-164, which is what added the manifest and the 178 keys). Its component edits are therefore stale, and the overlap is worth checking before merge rather than after.
• it has had 0 reviews and 0 comments since 2026-09-16.

[lilyshen0722]

Second cross-reference, and this one is a warning rather than a status note: two of the six conflicts must not be resolved to 'theirs', or this PR silently reverts copy that was ruled after it was written.

Measured today against main 827b47e4 (this branch is 244 commits behind, merge-base 4e8de868):

• backend/integrations/manifests.ts — this PR carries the Telegram description 'One Telegram chat, one pod.' (authored 2026-09-16). Main's current copy is 'Link your Telegram chat to Commonly — every pod you turn on reaches it.', which landed 2026-09-25 in fix(tools): a phone keeps the connector row's reason, and the row speaks in the person's words #1874. TASK-154 reversed the one-chat-one-pod model, and fix(connectors): the Telegram tagline teaches the model TASK-154 ruled, not one pod (TASK-158) #1957 (2026-09-27) added a main-side test — does not teach the one-pod model TASK-154 reversed — asserting the catalog description must not match /one pod/i. This PR's own new test in manifestReadiness.test.js asserts descriptions equals exactly that reversed sentence, so the two cannot both pass. Keep main's English and attach the zh-CN map beside it.
• backend/services/installable/toolInstallables.ts — same shape for GitHub: this PR re-authors the sentence (…called through Commonly's broker on a room grant.) against main's current …Connecting your own GitHub is coming. (also 2026-09-25, fix(tools): a phone keeps the connector row's reason, and the row speaks in the person's words #1874), and the zh-CN value here translates this PR's version rather than main's.

The remaining conflicts are mechanical: frontend/src/v2/components/V2ConnectorTools.tsx, V2ConnectorsPage.tsx and V2ConnectorsPage.test.tsx conflict only on adjacent lines (imports, a 2-line sibling, an appended it() block), and this PR's localizeInstallableDescription.ts + test are new files that merge clean.

The mechanism here is what's wanted; the English (and therefore the Chinese beside it) in those two backend files is stale. Also noted on the pod board, TASK-179.