feat(settings): clean up Jobs page, drop dead toggle, POC dirty-tracking action bar

[larsgeorge-db]

Summary

Three related Settings UX changes, all frontend-only.

1. General Settings cleanup

• Removed the Enable Background Jobs toggle from general-settings.tsx. It was a dead control: the backend never read or persisted enable_background_jobs, so flipping it changed nothing other than local React state.
• Moved the Workspace Deployment Path input from General Settings to the Jobs Configuration page (under Job Cluster ID), where it belongs. The path actually governs workflow installation, so it now lives next to the cluster/workflow controls it pairs with.
• i18n keys for the path field moved from settings.general.workspaceDeploymentPath.* to settings.jobs.workspaceDeploymentPath.* across all seven shipped locales (en, de, nl, fr, es, it, ja). One new key (requiredHint) added per locale.
• Added a path-required guard on the Jobs page: per-workflow Enable switches are disabled for currently-off rows when the deployment path is empty (already-on rows stay clickable for recovery), and Save is blocked with a destructive toast when any workflow is enabled but the path is empty. Inline Alert rendered under the path input when empty.

2. POC: dirty-tracking + Save/Cancel action bar on the Jobs page

• New shared component SettingsActionBar (src/frontend/src/components/settings/settings-action-bar.tsx): sticky bottom bar that renders only when the page is dirty. Save (primary, spinner while saving) + Cancel (outline, calls a caller-supplied revert) + an "unsaved changes" status pill.
• New shared component UnsavedChangesGuard (src/frontend/src/components/common/unsaved-changes-guard.tsx): installs a beforeunload listener while isDirty is true. Catches reload, tab close, and address-bar nav. Does not block in-app <Link> clicks (see Caveat below).
• jobs-settings.tsx migrated to use both: a JobsSnapshot (cluster id, deployment path, sorted enabled-job ids) is captured at load time, compared via a stable snapshotKey() to derive isDirty, restored on Cancel, and refreshed after a successful Save. The old Save button in DataTable.toolbarActions is gone.

3. PRD #428 for the broader rollout

• New doc docs/prds/prd-settings-form-save-cancel.md, filed as issue PRD: Consistent Save / Cancel and dirty-tracking across Settings form sub-views #428. Captures the plan to roll the dirty-tracking convention out across all six form-style settings sub-views (general, ui-customization, git, delivery, jobs, search-config-editor) with a static bottom-left Save/Cancel block — replacing this POC's sticky right-aligned bar, per the agreed convention.
• Out of scope of PRD: Consistent Save / Cancel and dirty-tracking across Settings form sub-views #428: list/CRUD settings pages (edits happen inside Dialogs), and the BrowserRouter → Data Router migration needed for in-app navigation blocking via useBlocker.

Caveats

• In-app navigation is not blocked. The app currently uses BrowserRouter (declarative), and useBlocker requires createBrowserRouter + RouterProvider. The visible action bar plus beforeunload is judged sufficient for now; full in-app blocking is tracked in PRD: Consistent Save / Cancel and dirty-tracking across Settings form sub-views #428's out-of-scope section.
• The POC bar in this PR is sticky and right-aligned — the bar style proven in this PR will be replaced with a static bottom-left block when PRD: Consistent Save / Cancel and dirty-tracking across Settings form sub-views #428 lands across all six pages. This PR ships the POC; PRD: Consistent Save / Cancel and dirty-tracking across Settings form sub-views #428 ships the final convention.

Backend impact

None. SettingsManager.update_settings already persisted workspace_deployment_path and triggers _reinitialize_workspace_deployer, so the moved input continues to work end-to-end without API changes.

Test plan

• General Settings: confirm the Background Jobs toggle and Workspace Deployment Path input are gone; remaining sections (Unity Catalog, Audit Log, LLM) save correctly.
• Jobs Settings:
  • Load page; confirm Job Cluster ID, Workspace Deployment Path, then the workflows table render in that order.
  • Clear the deployment path; confirm per-workflow Enable switches grey out for currently-off rows but stay clickable for currently-on rows; the inline Alert appears under the path input.
  • Enable a workflow with the path empty, click Save; confirm the destructive toast blocks the save.
  • Set a path, save; reload the page; confirm values persisted.
  • Edit any field; confirm the sticky action bar appears with the "unsaved changes" pill, Save, and Cancel.
  • Click Cancel; confirm every edited field reverts to the loaded value and the bar disappears.
  • Edit a field, hit browser reload; confirm the native "unsaved changes" prompt fires.
• i18n: switch language to each of de/nl/fr/es/it/ja; confirm the Jobs page renders the workspace path label + help in the locale; confirm General Settings no longer references background jobs / deployment path strings.
• TypeScript: tsc --noEmit clean (verified locally).

Refs #428