Translate the connector setup and new connector pages (DE, IT) - #1318

Summary

Fourth area of #1271, second half: the guided connector setup (/connectors/setup/[slug]) and the custom connector form (/connectors/new) in German and Italian, with the components they use. It follows #1303 (list and marketplace) and #1315 (card fix). No behaviour changes, and Accept-Language: en-US renders exactly what it rendered before.

Changes

• app/connectors/setup/[slug]/page.tsx: the setup form:
  • the field groups and states (required, looks wrong, optional, stored, show/hide);
  • the check results and their actions (save anyway, set up the other connector);
  • the OAuth notes and the callback URL box;
  • the guided-step buttons;
  • the draft hint and the error fallbacks.
• components/connector-setup/setup-guide.tsx: the guide intro (title, step count, what you need, why, resume / start over / skip), the stepper, "Step N of M", the values to copy, the screenshot caption and "Common problems".
• components/connector-setup/setup-success.tsx: the success screen:
  • the connected / verified lines and the way back to a chat;
  • the "Try asking" examples;
  • the per-client steps for Claude, ChatGPT and Copilot, with the same wording as the Quick Connect steps from Translate the MCP server pages (DE, IT) #1283;
  • the API-key-only and localhost warnings.
• app/connectors/new/page.tsx: the custom connector form:
  • the type cards and the marketplace card;
  • the catalog match;
  • the OData, database, spec, authentication, OAuth, API key, WS-Security and login-token sections;
  • the test-connection results;
  • the create and test buttons.
• components/headers-editor.tsx and components/mcp-assign-modal.tsx: shared with /connectors/[id], so they're translated here, as agreed in Translate the rest of the admin UI (DE, IT) #1271. HeadersEditor still takes a custom label/hint. Without one, it now uses the translated default instead of the English one.
• lib/use-effort-label.ts (new): the "API key · 3 min" badge in the UI language, now used by both the marketplace and the setup guide. In Translate the connector list and marketplace (DE, IT) #1303 the marketplace had its own copy of it; this replaces that copy and removes the English-only effortLabel from lib/setup-effort.ts.
• messages/{en,de,it}.json: new namespaces connectorSetup, setupGuide, setupSuccess, headersEditor, mcpAssign and connectorNew, with 231 keys each.

Left in English on purpose:

• Catalog content from the backend: adapter names, descriptions, field labels and help, prerequisites, the guide steps, values, problems and example prompts. This is the same rule as for other server text.
• Other apps' own buttons and menus in the client steps (Add custom connector, Customize → Connectors, Add custom MCP server, Server URL, Authentication, Create as a plugin, Tools), as in Translate the MCP server pages (DE, IT) #1283.
• Technical names on the custom form: Client ID, Client Secret, Token URL, Scopes, Bearer Token, WS-Security, PasswordText/PasswordDigest, POST/GET, plus the example values in placeholders.
• TrialLimitNotice, with the other billing components.
• The support email subject in "Stuck? Ask us".

Self-hosted vs Cloud: frontend only, with no backend, database or ee/ change.

Translations

German and Italian aren't my native languages, so please review those strings. I kept the tone and terms from the earlier PRs: German with Sie, Italian with tu, "Connector" / "connettore", "KI-Client" / "client AI", "Marktplatz" / "Marketplace". Strings I'm least sure about:

• Field groups: "Adresse / Zugangsdaten / Einstellungen" and "Indirizzo / Credenziali / Impostazioni" for "Where it is / Credentials / Settings".
• The last stepper item: "Nutzen" / "Usalo" for "Use it".
• The OAuth grant names: "Authorization Code", "Client Credentials" and "Password" are mostly kept as the OAuth terms, with a translated note in brackets.
• "SAP-Mandant" / "Mandante SAP" for "SAP client".
• The "Add to Claude" step: step 1 names our own button, so it uses the translated label ("Klicken Sie auf Zu Claude hinzufügen"). The dialogs it opens keep Claude's English names.

	German	Italian
Guide intro
Guide step
Success
/connectors/new

Type

• New feature

Testing

3 new tests in tests/e2e/i18n.spec.ts, with a mocked catalog connector (a two-step guide) and the adapter check and import:

• de-DE: the guided setup runs in German from the intro ("API-Schlüssel · 3 Min.", "2 Schritte") through a required-field error ("Pflichtfeld") to the success screen ("Shopx ist verbunden").
• it-IT: /connectors/new renders in Italian, including the REST form, the custom headers editor and the database access mode.
• en-US: both pages stay exactly as before.

The key-parity test from #1248 covers the new keys. The existing specs that check English text on these pages (guided setup, setup guide, catalog URL match, setup link, starter pack, mobile) all still pass.

The backend isn't touched, so I didn't rerun its suite.

• Added/updated tests
• Tested manually. I checked both flows in German and Italian in the browser with mocked API data (the screenshots above), not against a live backend.

Checklist

• My code follows the existing code style
• I have tested my changes locally
• I have updated documentation (if applicable). Not needed: CONTRIBUTING's "Translating the UI" section already covers new namespaces.
• All existing tests pass (frontend; backend unchanged)
• This PR has a descriptive title

packages/frontend/AGENTS.md and next-env.d.ts were rewritten by next dev locally and are left out, as AGENTS.md asks.

Related Issues

Part of #1271 (Connectors, second half). After this, the remaining areas are connector detail, settings and admin, and knowledge graph and memory.