feat(web): admin playback-errors inbox

Surfaces client-reported playback failures from /api/admin/playback-errors
in a new admin tab. Tabs: Unresolved (default) / Resolved. Each row
shows track + artist + album, error kind badge, who hit it, when,
optional client-supplied detail, and the absolute file path so the
operator can grep the library mount without leaving the page.

Per-row actions (RowActionsMenu):
- Resolve (primary) — modal with Fixed / Ignored dropdown for the
  "no further action taken" cases.
- Copy — JSON payload to clipboard with track_id / file_path / kind
  / detail / reporter / client_id / occurred_at. Matches the
  operator's "logs with a copy-out function" ask.
- Delete file (danger, modal-confirm) — uses the existing
  /api/admin/quarantine/{track_id}/delete-file endpoint AND
  auto-stamps resolution='deleted' so a single click closes both
  the file and the inbox row.

Deferred to a follow-up: Hide (the existing quarantine flow is
per-user-flag, not a true library-hide), and Re-request via Lidarr
(needs album MBID join — not in the current ListAdminPlaybackErrors
projection).

Also: admin tab list grows from five to six; AdminTabs.test.ts
updated.

web/src/lib/components/AdminTabs.svelte

@@ -4,11 +4,12 @@
   type Item = { href: string; label: string };
 
   const items: Item[] = [
-    { href: '/admin',              label: 'Overview'      },
-    { href: '/admin/integrations', label: 'Integrations'  },
-    { href: '/admin/requests',     label: 'Requests'      },
-    { href: '/admin/quarantine',   label: 'Quarantine'    },
-    { href: '/admin/users',        label: 'Users'         }
+    { href: '/admin',                 label: 'Overview'        },
+    { href: '/admin/integrations',    label: 'Integrations'    },
+    { href: '/admin/requests',        label: 'Requests'        },
+    { href: '/admin/quarantine',      label: 'Quarantine'      },
+    { href: '/admin/playback-errors', label: 'Playback errors' },
+    { href: '/admin/users',           label: 'Users'           }
   ];
 
   function isActive(href: string): boolean {

web/src/lib/components/AdminTabs.test.ts

@@ -52,7 +52,7 @@ describe('AdminTabs', () => {
     );
   });
 
-  test('renders exactly five tabs', () => {
+  test('renders all six tabs in order', () => {
     state.pageUrl = new URL('http://localhost/admin');
     render(AdminTabs);
     const links = screen.getAllByRole('link');
@@ -61,6 +61,7 @@ describe('AdminTabs', () => {
       'Integrations',
       'Requests',
       'Quarantine',
+      'Playback errors',
       'Users'
     ]);
   });