fix(mobile): allow stalled composer uploads to be cancelled

mobile/lib/features/channels/compose_bar.dart

@@ -546,7 +546,7 @@ class ComposeBar extends HookConsumerWidget {
           attachments.value = [...attachments.value, uploaded];
         }
       } catch (error) {
-        if (context.mounted) {
+        if (context.mounted && _shouldSurfaceUploadError(error)) {
           uploadError.value = _formatUploadError(error);
         }
       } finally {
@@ -566,12 +566,18 @@ class ComposeBar extends HookConsumerWidget {
         if (picked == null || !context.mounted) return;
         await pickAndUpload(() => upload(picked));
       } catch (error) {
-        if (context.mounted) {
+        if (context.mounted && _shouldSurfaceUploadError(error)) {
           uploadError.value = _formatUploadError(error);
         }
       }
     }
 
+    void cancelPendingUploads() {
+      ref.read(mediaUploadServiceProvider).cancelActiveUploads();
+      uploadingCount.value = 0;
+      uploadError.value = null;
+    }
+
     Future<void> uploadImages(List<XFile> images) async {
       if (images.isEmpty) return;
       uploadError.value = null;
@@ -617,7 +623,7 @@ class ComposeBar extends HookConsumerWidget {
             .map((result) => result.error)
             .whereType<Object>()
             .firstOrNull;
-        if (firstError != null) {
+        if (firstError != null && _shouldSurfaceUploadError(firstError)) {
           uploadError.value = _formatUploadError(firstError);
         }
       } finally {
@@ -727,7 +733,7 @@ class ComposeBar extends HookConsumerWidget {
         try {
           await choose();
         } catch (error) {
-          if (context.mounted) {
+          if (context.mounted && _shouldSurfaceUploadError(error)) {
             uploadError.value = errorMessage ?? _formatUploadError(error);
           }
         }
@@ -951,6 +957,8 @@ class ComposeBar extends HookConsumerWidget {
           uploadingCount: uploadingCount.value,
           onRemoveAttachment: removeAttachment,
           uploadError: uploadError.value,
+          onDismissUploadError: () => uploadError.value = null,
+          onCancelUpload: hasPendingUploads ? cancelPendingUploads : null,
           isExpanded: isComposerExpanded.value,
           controller: controller,
           focusNode: focusNode,

mobile/lib/features/channels/compose_bar/attachments.dart

@@ -471,11 +471,13 @@ class _AttachmentStrip extends StatelessWidget {
   final List<BlobDescriptor> attachments;
   final int uploadingCount;
   final void Function(String url) onRemove;
+  final VoidCallback? onCancelUpload;
 
   const _AttachmentStrip({
     required this.attachments,
     required this.uploadingCount,
     required this.onRemove,
+    this.onCancelUpload,
   });
 
   @override
@@ -539,6 +541,33 @@ class _AttachmentStrip extends StatelessWidget {
                           ),
                         ),
                       ),
+                    if (onCancelUpload != null)
+                      PositionedDirectional(
+                        top: 0,
+                        end: 0,
+                        child: Semantics(
+                          button: true,
+                          label: 'Cancel upload',
+                          child: Material(
+                            color: context.colors.surface.withValues(
+                              alpha: 0.92,
+                            ),
+                            shape: const CircleBorder(),
+                            clipBehavior: Clip.antiAlias,
+                            child: InkWell(
+                              onTap: onCancelUpload,
+                              child: Padding(
+                                padding: const EdgeInsets.all(4),
+                                child: Icon(
+                                  LucideIcons.x,
+                                  size: 14,
+                                  color: context.colors.onSurfaceVariant,
+                                ),
+                              ),
+                            ),
+                          ),
+                        ),
+                      ),
                   ],
                 ),
               ),

mobile/lib/features/channels/compose_bar/layout.dart

@@ -5,6 +5,8 @@ class _ComposeBarLayout extends StatelessWidget {
   final int uploadingCount;
   final ValueChanged<String> onRemoveAttachment;
   final String? uploadError;
+  final VoidCallback? onDismissUploadError;
+  final VoidCallback? onCancelUpload;
   final bool isExpanded;
   final TextEditingController controller;
   final FocusNode focusNode;
@@ -33,6 +35,8 @@ class _ComposeBarLayout extends StatelessWidget {
     required this.uploadingCount,
     required this.onRemoveAttachment,
     required this.uploadError,
+    this.onDismissUploadError,
+    this.onCancelUpload,
     required this.isExpanded,
     required this.controller,
     required this.focusNode,
@@ -81,18 +85,42 @@ class _ComposeBarLayout extends StatelessWidget {
               attachments: attachments,
               uploadingCount: uploadingCount,
               onRemove: onRemoveAttachment,
+              onCancelUpload: onCancelUpload,
             ),
             const SizedBox(height: Grid.xxs),
           ],
           if (uploadError case final error?) ...[
-            Align(
-              alignment: Alignment.centerLeft,
-              child: Text(
-                error,
-                style: context.textTheme.bodySmall?.copyWith(
-                  color: context.colors.error,
+            Row(
+              crossAxisAlignment: CrossAxisAlignment.start,
+              children: [
+                Expanded(
+                  child: Text(
+                    error,
+                    style: context.textTheme.bodySmall?.copyWith(
+                      color: context.colors.error,
+                    ),
+                  ),
                 ),
-              ),
+                if (onDismissUploadError != null)
+                  Semantics(
+                    button: true,
+                    label: 'Dismiss upload error',
+                    child: IconButton(
+                      visualDensity: VisualDensity.compact,
+                      padding: EdgeInsets.zero,
+                      constraints: const BoxConstraints(
+                        minWidth: 32,
+                        minHeight: 32,
+                      ),
+                      onPressed: onDismissUploadError,
+                      icon: Icon(
+                        LucideIcons.x,
+                        size: 16,
+                        color: context.colors.error,
+                      ),
+                    ),
+                  ),
+              ],
             ),
             const SizedBox(height: Grid.xxs),
           ],

mobile/lib/features/channels/compose_bar/send_button.dart

@@ -43,5 +43,14 @@ class _SendButton extends StatelessWidget {
 }
 
 String _formatUploadError(Object error) {
+  if (error is MediaUploadCancelledException) {
+    return '';
+  }
+  if (error is MediaUploadTimeoutException) {
+    return error.toString();
+  }
   return error.toString().replaceFirst('Exception: ', '');
 }
+
+bool _shouldSurfaceUploadError(Object error) =>
+    error is! MediaUploadCancelledException;