feat(web/m7-recurring-scans): admin Scan schedule section

Adds a "Scan schedule" section to the admin overview page between
the existing Library Scan and Cover Art sections. Mode picker
(Off / Every N hours / Daily / Weekly) with conditional interval /
time / day inputs. Save button disabled until local state diverges
from the server state. "Next scan: in N hours (timestamp)"
inline indicator when mode != off.

formatRelative is a static helper (recomputes on render); operator
sees the configuration confirmation at a glance — not a countdown
timer.

admin.test.ts mock extended with createScanScheduleQuery returning
mode='off' default. New test asserts the section renders. Existing
assertions still pass.

web/src/routes/admin/admin.test.ts

@@ -41,7 +41,21 @@ vi.mock('$lib/api/admin', async () => {
     deleteQuarantineFile: vi.fn().mockResolvedValue({}),
     deleteQuarantineViaLidarr: vi.fn().mockResolvedValue({}),
     triggerScan: vi.fn().mockResolvedValue({}),
-    refetchMissingCovers: vi.fn().mockResolvedValue({ queued: 0 })
+    refetchMissingCovers: vi.fn().mockResolvedValue({ queued: 0 }),
+    createScanScheduleQuery: vi.fn(() =>
+      readable({
+        data: {
+          mode: 'off',
+          interval_hours: null,
+          time_of_day: null,
+          weekly_day: null,
+          next_scheduled_at: null,
+        },
+        isPending: false,
+        isError: false
+      })
+    ),
+    updateScanSchedule: vi.fn().mockResolvedValue({})
   };
 });
 
@@ -242,6 +256,14 @@ describe('admin Overview page', () => {
     );
   });
 
+  test('renders the scan-schedule section with mode=off default', () => {
+    setupOverview();
+    render(OverviewPage);
+    expect(screen.getByText('Scan schedule')).toBeInTheDocument();
+    const select = screen.getByLabelText('Mode') as HTMLSelectElement;
+    expect(select.value).toBe('off');
+  });
+
   test('Library scan card shows Processing badge when library tally is populated and in-flight', async () => {
     const { readable } = await import('svelte/store');
     (createScanStatusQuery as ReturnType<typeof vi.fn>).mockReturnValueOnce(