feat(widgets): add frontend widget runtime (types, API, hooks, registry, components)

PR 3 of 4 for configurable dashboard widgets.

- Add TypeScript widget interfaces (WidgetInstance, WidgetInstanceInput,
  WidgetTypeInfo, WidgetDataResponse).
- Create widget API client for CRUD, registry metadata, and per-widget data.
- Create TanStack Query hooks for instances, data, sources, types, and mutations.
- Create closed frontend widget registry with metadata, source type, refresh
  intervals, and config fields.
- Add six shadcn/ui-based widget components: Jellyfin, Backups, Grafana link,
  Prometheus metric, SSH task output, and static text.
- Add Vitest unit tests for registry metadata.

Verification:
- backend ruff clean; pytest 200 passed
- frontend npm run lint: 0 errors
- frontend npm run build: success
- frontend npm run test -- src/widgets/registry.test.ts: 3 passed

openspec/changes/configurable-dashboard-widgets/apply-progress.md

@@ -93,9 +93,56 @@ Focused widget test output: `27 passed`.
 
 - Adapters currently call `get_settings_store()` internally for `backups`/`ssh_task` sources. The router-level endpoint uses FastAPI DI, but adapter unit tests patch `get_settings_store` to inject a test store. A future refactor can pass `store` and `settings` explicitly into `adapter.fetch()` for cleaner testability.
 
+## Completed tasks (Slice 3)
+
+All Slice 3 tasks are marked `- [x]` in `tasks.md`:
+
+- [x] 3.1 Add TypeScript widget interfaces (`WidgetInstance`, `WidgetInstanceInput`, `WidgetTypeInfo`, `WidgetDataResponse`)
+- [x] 3.2 Create widget API client (`frontend/src/api/widgets.ts`)
+- [x] 3.3 Create widget TanStack Query hooks (`frontend/src/hooks/useWidgets.ts`)
+- [x] 3.4 Create frontend widget registry (`frontend/src/widgets/registry.ts`)
+- [x] 3.5 Implement six widget presentational components (`frontend/src/widgets/*.tsx`)
+- [x] 3.6 Add frontend registry unit test (`frontend/src/widgets/registry.test.ts`)
+
+## Files changed (Slice 3)
+
+### New files
+
+- `frontend/src/api/widgets.ts` — API functions for widget CRUD, registry metadata, and per-widget data.
+- `frontend/src/hooks/useWidgets.ts` — TanStack Query hooks for instances, data, sources, types, and mutations.
+- `frontend/src/widgets/registry.ts` — Closed frontend registry with metadata, refresh intervals, and config fields.
+- `frontend/src/widgets/JellyfinWidget.tsx` — Renders Jellyfin session activity.
+- `frontend/src/widgets/BackupsWidget.tsx` — Renders backup dashboard summary.
+- `frontend/src/widgets/GrafanaLinkWidget.tsx` — Renders a deep-link to Grafana (no iframe).
+- `frontend/src/widgets/PrometheusMetricWidget.tsx` — Renders PromQL instant query result.
+- `frontend/src/widgets/SshTaskWidget.tsx` — Renders saved SSH task output.
+- `frontend/src/widgets/StaticWidget.tsx` — Renders static text.
+- `frontend/src/widgets/index.ts` — Barrel exports.
+- `frontend/src/widgets/registry.test.ts` — Vitest unit tests for registry metadata.
+
+### Modified files
+
+- `frontend/src/types/index.ts` — Added widget TypeScript interfaces.
+
+## Verification (Slice 3)
+
+```bash
+cd backend
+.venv/bin/python -m ruff check .              # All checks passed
+PYTHONPATH=src .venv/bin/python -m pytest     # 200 passed, 2 warnings
+cd ../frontend
+npm run lint                                  # 2 pre-existing warnings, 0 errors
+npm run build                                 # Built successfully
+npm run test -- src/widgets/registry.test.ts  # 3 passed
+```
+
+## Deviations from design (Slice 3)
+
+- Registry unit test is colocated at `frontend/src/widgets/registry.test.ts` and runs with Vitest, matching the project's existing `npm run test` setup, instead of `frontend/tests/widgets.test.mjs`.
+- `JellyfinWidget` uses `SessionActivityPanel` directly because `NowPlaying` does not expose an `emptyMessage` prop.
+
 ## Remaining work
 
-- Slice 3: Frontend types/API/hooks/registry/components
 - Slice 4: Dashboard loop + configuration UI + addon pages
 
 ## PR boundary