feat(surveys): edit survey settings after creation (Issue 1459)

[leahpeker]

Summary

• Adds a "settings" dialog on the survey builder screen to edit title, description, slug, visibility, one-response-per-user, and linked event via the existing PATCH /surveys/{id}/ endpoint (useUpdateSurvey).
• Extracts the shared field layout from CreateSurveyDialog into a new SurveyFields component so create and edit stay in sync; the linked-event picker reuses the existing useEvents hook and keeps a past/cancelled linked event selectable even once it drops off the active list.
• Slug collisions from the backend (Code.Survey.SlugAlreadyExists) now surface as a field-level error via a new getFieldError helper in apiErrors.ts, instead of the generic form alert.

Closes #1459

Test plan

• make agent-frontend-lint — passes
• make agent-frontend-typecheck — passes
• make agent-frontend-test — passes (1286 passed, 1 skipped)
• make agent-ci — backend ruff, manage.py check, ty typecheck, cognitive-complexity, and check-codes all pass clean. The backend pytest run has 7 pre-existing failures unrelated to this change (this branch is frontend-only): 5 test_sse.py auth tests that need Postgres pg_notify (SSE is documented in CLAUDE.md as returning 503 on SQLite), and 2 concurrency race tests (test_poll_finalize_race.py, test_rsvp_capacity_race.py) that hit SQLite's lack of real row-level locking under select_for_update.
• added SurveySettingsDialog.test.tsx covering: seeding fields from the survey and saving edits, keeping a non-active linked event selectable, surfacing a slug collision as a field error, and blocking save on a blank title.

[leahpeker]

Merge conflicts with main need manual resolution.

Rebasing feat-survey-settings-edit onto main surfaced conflicts in two commits:

Auto-resolvable (mechanical):

• frontend/src/screens/admin/SurveyAdminListScreen.tsx — import-only conflict; both sides added a different local import (SurveyCopyLinkButton from feat(surveys): add copyable participant link (Issue 1458) #1469 vs SurveyFields/useSurveyFieldErrors from this PR). Union of both.

Needs human judgment (blocker):

• frontend/src/screens/admin/SurveyAdminListScreen.test.tsx — add/add conflict. main (via feat(surveys): add copyable participant link (Issue 1458) #1469) and this PR each added a different test file at this path, with incompatible module-level setup:

  • main: mocks sonner, seeds useAdminSurveys with two surveys, tests the copy-link button.
  • this PR: mocks react-router-dom (useNavigate) and @/api/events, seeds useAdminSurveys with [], tests create-dialog reset + field-error routing.

  The two suites can't be concatenated — the vi.mock factories for @/api/surveyAdmin conflict, and the copy-link test depends on a non-empty survey list while the create tests assume an empty one. Merging them means picking one mock set and reconciling both suites against it.

Auto-resolution was skipped to avoid a risky merge; the branch was left untouched (rebase aborted).

[leahpeker]

Resolved — conflicts are gone, branch rebased onto main and force-pushed (fbacaa03).

Test file (the blocker): hand-merged into one file with a unified mock set — useAdminSurveys now seeds the two-survey fixture from main (the create-dialog tests don't depend on an empty list), useCreateSurvey uses the shared createMutateAsync spy, and the sonner, @/api/events, and react-router-dom mocks all coexist (the router mock spreads ...actual, so MemoryRouter/Link still work). Both suites kept as separate describe blocks sharing the renderScreen helper. All 6 tests pass.

Screen file: import-only union, as expected.

Two extra things while in here:

• Trimmed the verbose comments this PR added — dropped the two that restated the code below them, tightened the getFieldError docblock, kept the remount note and the linked-event constraint (both are real hidden constraints).
• SurveyFields.tsx was failing react-refresh/only-export-components by exporting useSurveyFieldErrors alongside the component (the config disables that rule only for src/router). Moved the hook to useSurveyFieldErrors.ts.

Verified: frontend lint, typecheck, and the full 1322-test suite pass; backend lint/typecheck/complexity pass. The 7 backend failures locally (SSE + the two race tests) are pre-existing SQLite-environment limits — identical on main.