Allow selecting and uploading multiple EPUB files at once

- views/pages.templ: upload input now has "multiple" attribute; page
  title/heading updated to plural ("Bücher hochladen"); templ regenerated
- internal/web/handlers.go: uploadSubmit now reads all files from
  r.MultipartForm.File["book"] instead of a single r.FormFile, saving
  each via a new saveUpload helper; partial failures are reported per
  filename while successfully saved files are kept

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

views/pages.templ

@@ -69,16 +69,16 @@ templ LoginPage(errMsg string) {
 }
 
 templ UploadPage(errMsg string) {
-  @Layout("Buch hochladen", "/") {
+  @Layout("Bücher hochladen", "/") {
     <section class="page-head">
-      <h2>Buch hochladen</h2>
+      <h2>Bücher hochladen</h2>
     </section>
     if errMsg != "" {
       <p class="error">{ errMsg }</p>
     }
     <form method="post" action="/upload" enctype="multipart/form-data" class="upload-form">
-      <label for="book">EPUB auswählen</label>
-      <input id="book" type="file" name="book" accept=".epub" required />
+      <label for="book">EPUB(s) auswählen</label>
+      <input id="book" type="file" name="book" accept=".epub" multiple required />
       <button type="submit" class="btn">Hochladen</button>
     </form>
   }