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/library/library.go

@@ -44,7 +44,7 @@ func (s *Service) ListBooks() ([]Book, error) {
 		}
 
 		ext := strings.ToLower(filepath.Ext(e.Name()))
-		if ext != ".epub" && ext != ".pdf" {
+		if ext != ".epub" {
 			continue
 		}
 
@@ -55,17 +55,15 @@ func (s *Service) ListBooks() ([]Book, error) {
 		hasCover := false
 		title := fallbackTitle
 
-		if ext == ".epub" {
-			meta, err := readEPUBMetadata(fullPath)
-			if err == nil {
-				if strings.TrimSpace(meta.Title) != "" {
-					title = strings.TrimSpace(meta.Title)
-				}
-				if strings.TrimSpace(meta.Author) != "" {
-					author = strings.TrimSpace(meta.Author)
-				}
-				hasCover = meta.HasCover
+		meta, err := readEPUBMetadata(fullPath)
+		if err == nil {
+			if strings.TrimSpace(meta.Title) != "" {
+				title = strings.TrimSpace(meta.Title)
 			}
+			if strings.TrimSpace(meta.Author) != "" {
+				author = strings.TrimSpace(meta.Author)
+			}
+			hasCover = meta.HasCover
 		}
 
 		books = append(books, Book{