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.

internal/router/frontend.go

@@ -12,6 +12,7 @@ import (
 	"strconv"
 	"time"
 
+	"bookhoard/internal/config"
 	"bookhoard/internal/database"
 	"bookhoard/internal/handlers"
 	"bookhoard/internal/services"
@@ -295,6 +296,18 @@ func registerFrontendRoutes(cfg *Config) {
 		libraryID := libRes.LibraryID
 		libData := libRes.Libraries
 
+		// Steam Deck behavior: an offline library (all folders missing, e.g.
+		// an unmounted external drive) shows an empty shelf with a notice
+		// instead of querying items. Nothing is marked or purged.
+		if !libRes.IsAll {
+			for _, l := range libData {
+				if l.ID == libraryID && l.Offline {
+					errorMsg = l.Name + " is currently unavailable - its storage is not connected. Your books are safe and will return when the drive is reattached."
+					break
+				}
+			}
+		}
+
 		// Fetch saved filters for SSR
 		userUUID, _ := uuid.Parse(user.ID)
 		var savedFilters []database.SavedFilters

internal/router/helpers.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"log"
 	"net/url"
+	"os"
 	"time"
 
 	"bookhoard/internal/database"
@@ -94,11 +95,11 @@ const selectedLibraryCookie = "selectedLibrary"
 const allLibrariesSentinel = "__all__"
 
 type LibraryResolution struct {
-	LibraryID  string
-	IsAll      bool
-	LibUUID    pgtype.UUID
-	Libraries  []templates.LibraryData
-	FirstID    string
+	LibraryID string
+	IsAll     bool
+	LibUUID   pgtype.UUID
+	Libraries []templates.LibraryData
+	FirstID   string
 }
 
 func getText(t pgtype.Text) string {
@@ -132,12 +133,30 @@ func resolveLibrary(c *echo.Context, cfg *Config, userUUID string) LibraryResolu
 	res.Libraries = make([]templates.LibraryData, len(libraries))
 	for i, lib := range libraries {
 		libUUID, _ := uuid.FromBytes(lib.ID.Bytes[0:16])
+		// Steam Deck SD-card behavior: a library whose folders are all
+		// missing (unmounted drive) renders offline - hidden contents, no
+		// marking, no purging - and returns when the storage does.
+		offline := false
+		folders, folderErr := cfg.Queries.GetLibraryFolders(c.Request().Context(), lib.ID)
+		if folderErr != nil || len(folders) == 0 {
+			offline = true
+		} else {
+			anyFolderExists := false
+			for _, folder := range folders {
+				if _, statErr := os.Stat(folder.FolderPath); statErr == nil {
+					anyFolderExists = true
+					break
+				}
+			}
+			offline = !anyFolderExists
+		}
 		res.Libraries[i] = templates.LibraryData{
 			ID:          libUUID.String(),
 			Name:        lib.Name,
 			Description: getText(lib.Description),
 			TypeName:    lib.TypeName,
 			MediaCount:  countMap[libUUID.String()],
+			Offline:     offline,
 		}
 	}