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>

views/pages.templ

@@ -77,8 +77,8 @@ templ UploadPage(errMsg string) {
       <p class="error">{ errMsg }</p>
     }
     <form method="post" action="/upload" enctype="multipart/form-data" class="upload-form">
-      <label for="book">EPUB oder PDF auswählen</label>
-      <input id="book" type="file" name="book" accept=".epub,.pdf" required />
+      <label for="book">EPUB auswählen</label>
+      <input id="book" type="file" name="book" accept=".epub" required />
       <button type="submit" class="btn">Hochladen</button>
     </form>
   }
@@ -104,7 +104,7 @@ templ IndexPage(books []library.Book, canUpload bool) {
       }
     </section>
     if len(books) == 0 {
-      <p>Keine eBooks gefunden. Lege EPUB- oder PDF-Dateien im Ordner <code>books/</code> ab.</p>
+      <p>Keine eBooks gefunden. Lege EPUB-Dateien im Ordner <code>books/</code> ab.</p>
     } else {
       <ul class="book-list">
         for _, b := range books {