feat(reader): comic treatment for fixed-layout items in comics/manga libraries

PDFs shelved in comics or manga libraries (scanned manga, official manga
PDFs with text layers) now get comic reader treatment: bookmarks only —
no text-selection highlights, no annotations, no in-book search. Items
in ebooks libraries are completely unaffected, and EPUBs keep their
existing behavior everywhere.

- handlers: new exported ShouldTreatAsComic(libraryType, formatGroup) —
  true for fixed_layout/comic_archive in manga/comics libraries; the
  reader JSON API also exposes it as treat_as_comic
- router: the SSR reader page fetches the library type and passes
  TreatAsComic through ReaderMetadata into the init config
- reader: when treatAsComic is set, the PDF textLayer selection listener
  (mouse and touch paths) never attaches so no highlight popover can
  open; pointer mode is forced to pan (Smart/Text segment hidden);
  search button and toggleSearch are disabled; previously-created PDF
  highlights stop rendering
- bookmarks are unchanged (already page-index based for fixed layout),
  and device sync / OPDS / format classification are untouched since
  format_group stays fixed_layout

templates/types.go

@@ -243,6 +243,9 @@ type ReaderMetadata struct {
 	LibraryID        string `json:"library_id"`
 	TotalCharacters  int64  `json:"total_characters"`
 	EstimatedPages   int    `json:"estimated_pages"`
+	// TreatAsComic: fixed-layout item in a comics/manga library gets
+	// comic reader treatment (bookmarks only — no highlights, no search).
+	TreatAsComic bool `json:"treat_as_comic"`
 }
 
 type ReadingProgress struct {