fix(ui): derive Library Type dropdowns from library_types — audiobooks was missing

The audiobook library type shipped backend-complete (LibraryTypeAudiobooks
constant, allowed extensions, MIME map, schema seed row, GET
/api/libraries/types) but neither create-library form offered it: the
admin Create Library modal and the setup wizard's add-library step both
hardcoded their <option> lists to the original three types
(ebooks/comics/manga), so an audiobook library could not be created from
the UI.

Both dropdowns now render from the same DB-backed library_types table
the create API resolves types against — a future type appears without
template edits (the exact drift the rendering-ownership rule warns
about):

- templates/utils.go: OrderedLibraryTypes pins the historical dropdown
  order (ebooks, comics, manga, audiobooks) and appends unknown future
  types by name; LibraryTypeDisplayName renders labels with a
  capitalize-the-name fallback for types added without a label
- templates/admin_library.templ: new LibraryTypeOptions fragment used
  by the modal's select; AdminLibrary takes the types list
- templates/setup.templ: Setup takes the types list, same fragment
- internal/router/helpers.go: libraryTypeOptionsFor fetches via
  LibraryService.GetLibraryTypes, falling back to the built-in type
  names on DB failure so the forms stay usable (CreateLibrary still
  resolves the name against the DB, so the fallback is never
  authoritative)
- internal/router/frontend.go, internal/router/setup.go: pass the list
  into the two pages

Tests:
- templates/utils_test.go: pure unit tests for ordering + labels
- cmd/server/tests/admin_library_page_test.go: SSR regression —
  /admin/library must offer all four library types, audiobooks exactly
  once (options come from the DB loop, not a hardcode)

templates/utils_test.go

@@ -1,6 +1,10 @@
 package templates
 
-import "testing"
+import (
+	"testing"
+
+	"bookhoard/internal/database"
+)
 
 func TestGroupTunableSettings(t *testing.T) {
 	entries := []SettingEntry{
@@ -54,3 +58,40 @@ func TestThemeCheckClass(t *testing.T) {
 		t.Errorf("inactive theme should be hidden, got %q", got)
 	}
 }
+
+// The library-type dropdown renders from DB rows; ordering and labels are
+// presentation policy (OrderedLibraryTypes / LibraryTypeDisplayName).
+func TestOrderedLibraryTypes(t *testing.T) {
+	// The DB query returns ORDER BY name; the dropdown must not reshuffle
+	// the historical UI order, and unknown (future) types append by name.
+	types := []database.LibraryTypes{
+		{Name: "audiobooks"}, {Name: "comics"}, {Name: "ebooks"},
+		{Name: "manga"}, {Name: "videos"},
+	}
+	got := OrderedLibraryTypes(types)
+	want := []string{"ebooks", "comics", "manga", "audiobooks", "videos"}
+	if len(got) != len(want) {
+		t.Fatalf("got %d types, want %d", len(got), len(want))
+	}
+	for i, name := range want {
+		if got[i].Name != name {
+			t.Errorf("position %d = %q, want %q", i, got[i].Name, name)
+		}
+	}
+}
+
+func TestLibraryTypeDisplayName(t *testing.T) {
+	cases := map[string]string{
+		"ebooks":     "Ebooks",
+		"comics":     "Comics",
+		"manga":      "Manga",
+		"audiobooks": "Audiobooks",
+		"videos":     "Videos", // unknown type: capitalized fallback
+		"":           "",
+	}
+	for name, want := range cases {
+		if got := LibraryTypeDisplayName(name); got != want {
+			t.Errorf("LibraryTypeDisplayName(%q) = %q, want %q", name, got, want)
+		}
+	}
+}