Drop PDF support, EPUB only

- internal/library: ListBooks only scans .epub files, dropping the
  pdf-specific metadata branch (epub metadata reading is now
  unconditional)
- internal/web: uploadSubmit rejects non-.epub uploads with updated
  error message
- views: upload form/file input and empty-library hint now reference
  only EPUB; templ regenerated
- README/copilot-instructions updated to reflect EPUB-only scope

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/web/handlers.go

@@ -210,8 +210,8 @@ func (h *Handler) uploadSubmit(w http.ResponseWriter, r *http.Request) {
 	defer file.Close()
 
 	ext := strings.ToLower(filepath.Ext(header.Filename))
-	if ext != ".epub" && ext != ".pdf" {
-		render(w, r.Context(), views.UploadPage("Nur EPUB- und PDF-Dateien erlaubt."))
+	if ext != ".epub" {
+		render(w, r.Context(), views.UploadPage("Nur EPUB-Dateien erlaubt."))
 		return
 	}