feat(web): admin page for files the library has lost — #2527

Renders GET /api/admin/library/missing under Admin -> Missing files.
Folder-grouped, because that is the unit an operator decides about: the
case behind #2523 was three reorganised albums, and forty individual
rows hides that it is really three decisions.

Each row leads with the fact that settles whether a missing file is
worth chasing -- "last played 2d ago" against "never played". The group
header carries how many tracks and how long they have been gone.

Read-only. No remove button anywhere: the row, its play history and its
likes survive a file going missing, and the scanner clears the mark by
itself when the file returns (or adopts the row if it returns renamed,
#2528). The page says so in its own copy rather than leaving the
operator to infer it.

Empty state explains the feature instead of the emptiness -- what puts a
row here (moved outside Minstrel, deleted, a drive that didn't mount)
and that rows leave on their own. Someone who has never seen this page
should not have to guess.

Paging follows the house pattern -- plain offset into the factory,
wrapped in $derived so a page change re-creates the query with a new
key. Passing a getter instead would capture the key once and paging
would silently not refetch. The pager only renders when it can do
something.

web/src/lib/components/AdminTabs.svelte

@@ -8,6 +8,7 @@
     { href: '/admin/integrations',    label: 'Integrations'    },
     { href: '/admin/requests',        label: 'Requests'        },
     { href: '/admin/quarantine',      label: 'Quarantine'      },
+    { href: '/admin/missing-files',   label: 'Missing files'   },
     { href: '/admin/playback-errors', label: 'Playback errors' },
     { href: '/admin/diagnostics',     label: 'Diagnostics'     },
     { href: '/admin/tuning',          label: 'Tuning'          },