feat(widgets): dashboard loop, widget config UI, and addon pages

PR 4 of 4 for configurable dashboard widgets.

- Replace hard-coded Jellyfin/Backups dashboard sections with a loop that
  renders enabled widget instances by sort_order.
- Add WidgetInstance renderer and WidgetConfigDialog for adding, editing,
  enabling/disabling, deleting, and reordering widgets.
- Add addon pages for grafana, prometheus, and ssh-tasks at /addons/:addonId.
- Register /addons/:addonId route in App.tsx.
- Update docs/REQUIREMENTS.md with the widget system design and API.

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

@@ -141,9 +141,55 @@ npm run test -- src/widgets/registry.test.ts  # 3 passed
 - 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.
 
+## Completed tasks (Slice 4)
+
+All Slice 4 tasks are marked `- [x]` in `tasks.md`:
+
+- [x] 4.1 Refactor `Dashboard.tsx` to render enabled widget instances in sort order
+- [x] 4.2 Create `WidgetInstance` renderer component
+- [x] 4.3 Create `WidgetConfigDialog` for add/edit/reorder/delete widgets
+- [x] 4.4 Create addon pages (`AddonPage`, `GrafanaAddonPage`, `PrometheusAddonPage`, `SshTasksAddonPage`)
+- [x] 4.5 Register `/addons/:addonId` route in `App.tsx`
+- [x] 4.6 Update `docs/REQUIREMENTS.md` with widget system documentation
+
+## Files changed (Slice 4)
+
+### New files
+
+- `frontend/src/components/WidgetInstance.tsx` — Renders a widget instance by looking up its definition and dispatching to the registered component.
+- `frontend/src/components/WidgetConfigDialog.tsx` — Dashboard widget configuration UI: list, add, edit, delete, reorder, enable/disable.
+- `frontend/src/pages/AddonPage.tsx` — Route mapper for `/addons/:addonId`.
+- `frontend/src/addons/GrafanaAddonPage.tsx` — Grafana addon landing page (deep-link only).
+- `frontend/src/addons/PrometheusAddonPage.tsx` — Prometheus addon landing page.
+- `frontend/src/addons/SshTasksAddonPage.tsx` — SSH tasks addon landing page.
+- `frontend/src/addons/index.ts` — Barrel exports.
+
+### Modified files
+
+- `frontend/src/pages/Dashboard.tsx` — Replaced hard-coded Jellyfin/Backups sections with widget instance loop; kept Shortcuts section; added "Edit dashboard" button.
+- `frontend/src/App.tsx` — Registered `/addons/:addonId` route in both OIDC and non-OIDC route trees.
+- `docs/REQUIREMENTS.md` — Added Configurable Dashboard Widgets section.
+
+## Verification (Slice 4)
+
+```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 4)
+
+- The "Edit dashboard" button lives in the Shortcuts section action area for now. A future UI pass can move it to a dedicated dashboard header.
+- Machine/task selectors in the config dialog filter to enabled Jellyfin machines / enabled tasks, which is slightly stricter than the design's generic string field.
+
 ## Remaining work
 
-- Slice 4: Dashboard loop + configuration UI + addon pages
+- Phase 1 widget system is complete. Future work could include widget grid layout, drag-and-drop reorder, richer Prometheus visualizations, or migrating shortcuts into the widget system.
 
 ## PR boundary