Let group admins delete the group's picture

Task #37: Add a "Remove picture" action next to the camera icon in the
group chat header so admins can clear a group's avatar back to the
default Users icon, not just replace it.

Implementation:
- artifacts/teaboy-os/src/pages/chat.tsx: Added a small X button overlay
  on the chat header avatar (positioned at -top-1 -end-1, mirrored for
  RTL). It only renders when the conversation is a group, the current
  user is an admin, and the group has an avatar set. Clicking it sends
  PATCH /api/conversations/:id with { avatarUrl: null } via the existing
  useUpdateConversation hook and invalidates the conversations list so
  both the chat header and conversation list fall back to the default
  Users fallback icon.
- Reused existing locale keys chat.removeAvatar (EN: "Remove picture",
  AR: "إزالة الصورة"), so it works in Arabic and English with no
  locale changes required.
- Backend (artifacts/api-server/src/routes/conversations.ts) and the
  generated UpdateConversationBody schema already accept avatarUrl:
  null, so no API or codegen changes were needed.

Notes / deviations:
- Pre-existing TypeScript errors in chat.tsx (codegen drift around
  useUpdateConversation, isMuted, isArchived, avatarUrl, etc.) are
  unrelated to this change; my new code follows the same patterns
  already used in this file and the dev server runs fine.

artifacts/teaboy-os/src/pages/chat.tsx

@@ -546,6 +546,32 @@ export default function ChatPage() {
                   )}
                 </label>
               )}
+              {selectedConv.isGroup && isAdminOfSelected && selectedConv.avatarUrl && (
+                <button
+                  type="button"
+                  onClick={() => {
+                    if (!selectedConvId) return;
+                    updateConv.mutate(
+                      { id: selectedConvId, data: { avatarUrl: null } },
+                      {
+                        onSuccess: () => {
+                          queryClient.invalidateQueries({ queryKey: getListConversationsQueryKey() });
+                        },
+                        onError: (err: Error) => {
+                          toast({ title: t("chat.settings.saveFailed"), description: err.message, variant: "destructive" });
+                        },
+                      },
+                    );
+                  }}
+                  disabled={isUploadingHeaderAvatar || updateConv.isPending}
+                  className="absolute -top-1 -end-1 w-5 h-5 rounded-full bg-white border border-slate-200 shadow-sm flex items-center justify-center text-slate-600 hover:text-destructive disabled:opacity-50"
+                  aria-label={t("chat.removeAvatar")}
+                  title={t("chat.removeAvatar")}
+                  data-testid="button-remove-group-avatar"
+                >
+                  <X size={10} />
+                </button>
+              )}
             </div>
           ) : (
             <MessageCircle size={20} className="text-green-400 shrink-0" />