feat(files): block upload and show warning for legacy .ppt/.doc file formats

- Set file_list_status to 'blocked_legacy' when any selected file is .ppt or .doc,
  disabling the Upload button until the file is removed
- Show a red banner at the top when upload is blocked
- Add a per-file warning message row in the file table for all legacy/untrusted
  extensions (previously computed but never rendered — only a pink cell highlight)
- Red styling for blocking extensions (.ppt/.doc), yellow for warn-only

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/elements/element_input_files_tbl.svelte

@@ -215,6 +215,10 @@ async function process_file_list(file_list: FileList) {
 
         file_data['warning_message'] = warning_message;
 
+        // .ppt and .doc must be converted before upload — block, don't just warn
+        file_data['block_upload'] =
+            guessed_extension === 'ppt' || guessed_extension === 'doc';
+
         file_data['uploaded'] = null;
         file_data['uploaded_bytes'] = null;
 
@@ -272,7 +276,8 @@ async function process_file_list(file_list: FileList) {
         // input_file_list_processed.push(file_data);
     }
 
-    file_list_status = 'ready';
+    const has_blocked = processed_file_list.some((f) => f.block_upload);
+    file_list_status = has_blocked ? 'blocked_legacy' : 'ready';
     console.log(processed_file_list);
 
     // return JSON.parse(JSON.stringify(processed_file_list));
@@ -361,6 +366,13 @@ run(() => {
         </div>
     {/if}
 
+    {#if file_list_status == 'blocked_legacy'}
+        <div class="m-1 rounded-md border-2 border-red-500 bg-red-100 p-3 text-sm text-red-800 dark:bg-red-900/30 dark:text-red-200">
+            <strong>⚠ Upload blocked — legacy file format detected.</strong>
+            Please remove the flagged file(s) below and re-save in the modern format before uploading.
+        </div>
+    {/if}
+
     <!-- {#await processed_file_list} -->
     <!-- {:then} -->
     {#if use_selected_file_table && processed_file_list && processed_file_list.length}
@@ -426,6 +438,19 @@ run(() => {
                             })}
                         </td>
                     </tr>
+                    {#if file_list_item.warning_message}
+                        <tr>
+                            <td colspan="6"
+                                class="text-left text-xs"
+                                class:bg-red-100={file_list_item.block_upload}
+                                class:text-red-800={file_list_item.block_upload}
+                                class:bg-yellow-50={!file_list_item.block_upload}
+                                class:text-yellow-900={!file_list_item.block_upload}>
+                                <!-- eslint-disable-next-line svelte/no-at-html-tags -->
+                                {@html file_list_item.warning_message}
+                            </td>
+                        </tr>
+                    {/if}
                 </tbody>
             {/each}
         </table>