fix(forum): Add FormData fallback for file upload on mobile

DataTransfer API may fail on some mobile browsers, leaving fileInput
empty. Added form submit interceptor that uses FormData as fallback
to ensure attachments are sent.

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

templates/forum/topic.html

@@ -2155,14 +2155,49 @@
         }
 
         function syncFilesToInput() {
-            // Create DataTransfer and add all files from Map
-            const dataTransfer = new DataTransfer();
-            filesMap.forEach(file => {
-                dataTransfer.items.add(file);
-            });
-            fileInput.files = dataTransfer.files;
+            try {
+                const dataTransfer = new DataTransfer();
+                filesMap.forEach(file => {
+                    dataTransfer.items.add(file);
+                });
+                fileInput.files = dataTransfer.files;
+            } catch (e) {
+                // DataTransfer not supported — form submit handler will use FormData fallback
+            }
         }
 
+        // Intercept form submit to ensure files are sent even if DataTransfer failed
+        const replyForm = dropzone.closest('form');
+        replyForm.addEventListener('submit', function(e) {
+            if (filesMap.size === 0) return; // no files, let form submit normally
+
+            // Check if fileInput actually has files (DataTransfer worked)
+            if (fileInput.files && fileInput.files.length > 0) return;
+
+            // DataTransfer failed — manually submit with FormData
+            e.preventDefault();
+            const formData = new FormData(replyForm);
+            // Remove empty attachments[] field
+            formData.delete('attachments[]');
+            // Add files from Map
+            filesMap.forEach(file => {
+                formData.append('attachments[]', file);
+            });
+
+            fetch(replyForm.action, {
+                method: 'POST',
+                body: formData
+            }).then(resp => {
+                if (resp.redirected) {
+                    window.location.href = resp.url;
+                } else {
+                    window.location.reload();
+                }
+            }).catch(() => {
+                showToast('Błąd wysyłania', 'error');
+            });
+        });
+
         function formatFileSize(bytes) {
             if (bytes < 1024) return bytes + ' B';
             if (bytes < 1024 * 1024) return (bytes / 1024).toFixed(1) + ' KB';