Cleanup: delete dead top-level pages + update docs (Slice 11)

Delete the old top-level page files whose content was migrated into
service-page tabs in slices 5-9:
- pages/Media.tsx, Applications.tsx (-> MediaTab)
- pages/FileBrowser.tsx, FileBrowser.impl.tsx (-> FilesTab)
- pages/Actions.tsx (-> ActionsTab)
- pages/Users.tsx, UsersPage.impl.tsx (replaced by Authentik tabs)
- components/BackupsPage.tsx (-> JobsTab)
- components/ObservabilityPage.tsx (split into Alerts/Links/Metrics tabs)
- hooks/useUsers.ts (orphaned after Users page deletion)
- the corresponding page test files (Media, FileBrowser, Applications,
  Actions, UsersPage) that tested the deleted pages directly.

The service-tab components are the live implementations; ServicePage
renders them. No live code references the deleted files.

Docs: append an Information Architecture section to REQUIREMENTS.md
documenting the services-as-hub model (nav shape, service-page tabs,
service type registry, Users->Authentik, Observability split, legacy
route 404s, empty state). Add a CHANGELOG entry under [Unreleased].

92 frontend tests pass (was 112; -20 deleted page tests); 271 backend
tests pass; lint/build green.

Refs openspec/changes/services-as-hub-ia/ (tasks slice 11).

.pi-tmp/followup1-wiring-out.md

@@ -0,0 +1,88 @@
+# Follow-up 1 — SheetForm isDirty wiring (worker output)
+
+## Task
+
+Wire the new `isDirty` prop of `SheetForm` into three remaining form consumers (Settings machine editor, message compose, WidgetConfigDialog) so unsaved edits trigger a "Discard changes?" confirm before closing.
+
+## Files changed (this worker's scope)
+
+| File | Status | Lines |
+|------|--------|-------|
+| `frontend/src/pages/Settings.tsx` | modified | +25 (isMachineDraftDirty helper + isDirty prop) |
+| `frontend/src/pages/UsersPage.impl.tsx` | modified | +5 (isDirty prop on compose SheetForm) |
+| `frontend/src/components/WidgetConfigDialog.tsx` | modified | +1 (isDirty prop) |
+| `frontend/src/pages/__tests__/Settings.test.tsx` | modified | +23 (dirty guard test) |
+| `frontend/src/pages/__tests__/UsersPage.test.tsx` | modified | +31 (compose dirty guard test) |
+| `frontend/src/components/__tests__/WidgetConfigDialog.test.tsx` | modified | +14 (draft dirty guard test) |
+
+**Total: ~99 changed lines** — well under the 250-line budget.
+
+## isDirty expressions per consumer
+
+### 1. Settings machine editor (`Settings.tsx`)
+
+Helper function `isMachineDraftDirty(draft, editingMachine)`:
+
+- **Create mode** (`editingMachine === null`): always dirty (return `true`).
+- **Edit mode**: field-by-field comparison of user-editable fields:
+  - `name`, `host`, `mode`, `port`, `username`, `ssh_key_id`, `enabled`, `notes`
+  - `services` array (sorted JSON.stringify comparison for order-insensitivity)
+
+```ts
+function isMachineDraftDirty(draft, editingMachine): boolean {
+  if (!editingMachine) return true;
+  return (
+    draft.name !== editingMachine.name ||
+    draft.host !== editingMachine.host ||
+    draft.mode !== editingMachine.mode ||
+    draft.port !== editingMachine.port ||
+    draft.username !== editingMachine.username ||
+    draft.ssh_key_id !== editingMachine.ssh_key_id ||
+    draft.enabled !== editingMachine.enabled ||
+    draft.notes !== editingMachine.notes ||
+    JSON.stringify([...draft.services].sort()) !==
+      JSON.stringify([...editingMachine.services].sort())
+  );
+}
+```
+
+Note: `node_exporter_scrape_host` (mentioned in the task) does not exist in either `MonitoringMachine` or `MonitoringMachineInput` in this codebase. The comparable editable fields were used instead. Secret fields (`ssh_private_key`, `password`) are excluded because they're write-only (the original only has `*_set` booleans, not values).
+
+### 2. Message compose (`UsersPage.impl.tsx`)
+
+```ts
+isDirty={
+  subject.trim() !== "" ||
+  htmlBody.trim() !== DEFAULT_HTML_BODY.trim() ||
+  attachments.length > 0
+}
+```
+
+### 3. WidgetConfigDialog (`WidgetConfigDialog.tsx`)
+
+```ts
+isDirty={draft !== null}
+```
+
+Dirty only in draft mode (when adding/editing a widget). In list mode, `draft === null` → `isDirty = false` (nothing to discard). In draft mode, `onCancel={reset}` returns to the list (does NOT close the sheet), so `isDirty` prompts before resetting the draft.
+
+## Validation
+
+```
+npm run lint   → 0 errors, 2 pre-existing warnings (UsersPage.impl.tsx exhaustive-deps)
+npm run build  → ✓ built (tsc -b + vite)
+npm run test   → 28 files / 122 tests passed (was 119; +3 new dirty-guard tests)
+```
+
+## Deviations from task
+
+1. **`node_exporter_scrape_host` field**: mentioned in the task but does not exist in the type definitions. Used the actual editable fields that exist on both `MonitoringMachine` and `MonitoringMachineInput`.
+2. **Secret fields excluded from dirty check**: `ssh_private_key`, `password`, `ssh_private_key_passphrase` are write-only on the draft and have no comparable value on `editingMachine` (which only has `*_set` booleans). Including them would make the form always dirty in edit mode.
+
+## skill_resolution
+
+`none` — no project/user SKILL.md paths were injected; no `.atl/skill-registry.md` found.
+
+## Residual risks
+
+- None for this worker's scope. The SheetForm primitive and ServicePage wiring were done by the parent and are not touched here.