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.go

@@ -409,3 +409,61 @@ func badgeLabel(badge string) string {
 	}
 	return ""
 }
+
+// Library type dropdown helpers. The option list comes from the DB-backed
+// library_types table (the same source the create API resolves types
+// against), so a new type appears in the UI without template edits.
+
+// libraryTypeDisplayOrder pins the long-standing library types to their
+// historical dropdown order.
+var libraryTypeDisplayOrder = map[string]int{
+	"ebooks":     0,
+	"comics":     1,
+	"manga":      2,
+	"audiobooks": 3,
+}
+
+const libraryTypeRankUnknown = 1 << 30
+
+func libraryTypeRank(name string) int {
+	if rank, ok := libraryTypeDisplayOrder[name]; ok {
+		return rank
+	}
+	return libraryTypeRankUnknown
+}
+
+// OrderedLibraryTypes sorts library types for a create-library dropdown:
+// the original types keep their UI order, any newer types follow by name.
+func OrderedLibraryTypes(types []database.LibraryTypes) []database.LibraryTypes {
+	sorted := make([]database.LibraryTypes, len(types))
+	copy(sorted, types)
+	sort.SliceStable(sorted, func(i, j int) bool {
+		ri, rj := libraryTypeRank(sorted[i].Name), libraryTypeRank(sorted[j].Name)
+		if ri != rj {
+			return ri < rj
+		}
+		return sorted[i].Name < sorted[j].Name
+	})
+	return sorted
+}
+
+// LibraryTypeDisplayName renders a library_types.name for the UI
+// ("audiobooks" → "Audiobooks"); unknown names just get their first letter
+// capitalized so a future type still reads sensibly.
+func LibraryTypeDisplayName(name string) string {
+	switch name {
+	case "ebooks":
+		return "Ebooks"
+	case "comics":
+		return "Comics"
+	case "manga":
+		return "Manga"
+	case "audiobooks":
+		return "Audiobooks"
+	}
+	if name == "" {
+		return name
+	}
+	r := []rune(name)
+	return strings.ToUpper(string(r[0])) + string(r[1:])
+}