feat(ui): hide missing books immediately and mark offline libraries

Two visibility fixes so the UI reflects the shelf's real state on the
next scan instead of only after the archive gate:

- Missing items disappear at once: the user-facing filters already hid
  archived rows; the same listings now also require missing_scan_count =
  0. A deleted or moved-then-not-yet-repointed file vanishes from the
  UI immediately, while purge timing stays gated on archived_at plus the
  retention window - grace protects data, not visibility. Restored
  automatically when the file returns.
- Steam Deck SD-card model for unmounted storage: resolveLibrary stats
  each library's folder roots and flags libraries with no live folder
  as Offline (LibraryData gains the field, computed at request time so
  mounts/unmounts react instantly). The bookshelf shows an empty shelf
  plus a 'storage is not connected' notice for an offline selected
  library, and both the shared LibrarySwitcher and the bookshelf's
  inline select label offline libraries with their true holding counts.
  Nothing is marked or purged while offline.
- TotalMediaCount skips offline libraries, so the 'All Books/Libraries'
  totals match what is actually visible.

Verified live: renaming uploads/Manga away produced the notice, an
empty shelf, and the offline dropdown label with a corrected total;
renaming it back restored all 38 cards with zero dirty rows.

templates/types.go

@@ -41,22 +41,40 @@ type LibraryData struct {
 	TypeValue   string
 	MediaCount  int64
 	FolderCount int
+	// Offline is true when none of the library's folders exist on disk
+	// (e.g. an unmounted external drive). Contents stay intact and reappear
+	// when the storage returns.
+	Offline bool
 }
 
 type FolderData struct {
 	FolderPath string
 }
 
+// ArchivedItem feeds the admin archived-items page: media hidden from
+// libraries because their files vanished, with their pending fate.
+type ArchivedItem struct {
+	ID           string
+	Title        string
+	Author       string
+	LibraryName  string
+	FilePath     string
+	MissingScans int32
+	ArchivedAt   *time.Time
+	PurgeAt      *time.Time
+	Status       string
+}
+
 type DirEntry struct {
 	Name string
 	Path string
 }
 
 type UserVisibilityData struct {
-	UserID     string
-	Username   string
-	Email      string
-	IsVisible  bool
+	UserID    string
+	Username  string
+	Email     string
+	IsVisible bool
 }
 
 type AdminStats struct {