fix(frontend): surface AI provider-required state and field-level validation errors (#177, #180)

[parthrohit22]

Summary

Two frontend honesty/UX fixes in the AI Workspace surface: (1) when no AI provider is configured, the page now shows a prominent notice linking to Settings instead of only a weak footer hint, so the Preview surface is not misleading about being inert (#177); (2) validation errors from the API are no longer collapsed into a generic "Request validation failed" string — field-level details from 422 responses are surfaced to the user (#180).

Linked issue

Related to #177 (AI Workspace inert state) and #180 (validation error collapse). Closes #177 and #180.

Roadmap alignment

• Roadmap §23 workstream this advances: W0 (Truth contract / honesty) + W4 (Product design / UX).
• §28 market-fit criterion this moves toward: "Trusted output" — product surfaces must state their actual state and never imply capability they lack (no misleading inert surfaces).
• Accepted evidence it is real: frontend lint clean, 176/176 vitest pass, build:frontend succeeds; the error box now renders getErrorDetail(...).details when present.

What changed

• apps/frontend/src/shared/services/api/errors.ts: getErrorMessage and new getErrorDetail extract field-level details from 422 bodies (FastAPI loc/msg shape) and append them instead of collapsing.
• apps/frontend/src/shared/services/api/index.ts: export getErrorDetail.
• apps/frontend/src/features/ai/hooks/useAIWorkspace.ts: fetch /ai/config on mount to expose providerConfigured.
• apps/frontend/src/app/pages/AIWorkspacePage.tsx: prominent amber "No AI provider configured → Settings" notice; error box renders structured field errors.
• apps/frontend/src/features/ai/hooks/useAIWorkspace.test.tsx: mock getConfig for the new mount effect.

Acceptance criteria completed

• Inert-without-provider state is explicitly shown (not just a footer hint).
• Validation (422) errors show field-level detail, not a generic collapse.
• Lint, vitest (176 passed), and frontend build all pass.
• No fabricated/interactive-without-backing behaviour introduced.

Testing performed

npm run lint:frontend            # clean
npm --prefix apps/frontend run test   # 176 passed (33 files)
npm run build:frontend           # built in ~480ms

Screenshots

Not applicable (logic/state change; no new visual assets). The provider-required notice reuses existing PreviewBanner-compatible styling.

Security and data considerations

None (client-side error presentation only; no secrets, no new API surface).

Dependencies and blocked work

None.

Scope changes or remaining work

#177's literal "reads legacy context" claim was already corrected in prior UI copy (it states sealed-snapshot structural facts). This PR adds the missing explicit provider-required state. No scope added beyond #177/#180.

Contributor checklist

• This PR targets dev
• The branch was created from an up-to-date upstream/dev
• This PR addresses one clearly scoped issue
• This PR advances a §23 workstream toward a §28 market-fit criterion (Roadmap alignment filled)
• Every acceptance criterion I claim as complete is actually complete
• Relevant tests pass
• Documentation is updated for any user-visible change
• No secrets, credentials, local env files, or generated artifacts are included
• No unrelated files were changed
• Closing syntax (Closes) is used only because the issue is fully resolved
• Dependencies and follow-up work are linked