fix(uploads): show the server's reason when a prompt upload fails

A failed upload only showed a count, so a remote session's refused upload read as a mystery instead of a rule the user could act on. The server already returns the reason in its error envelope and the per-file error carries it, so I surface the first one in the toast. One reason is enough when a whole batch fails for the same cause.

test/image-paste-trap.test.ts

@@ -238,4 +238,16 @@ describe('image upload insertion policy', () => {
     expect(app._uploadPasteImage).toHaveBeenCalledWith('session-b', { path: '/tmp/pane-b.png' });
     expect(app._sendInputAsync).toHaveBeenCalledWith('session-b', '/tmp/pane-b.png', { useMux: true });
   });
+
+  it('shows the server reason in the toast when an upload fails', async () => {
+    const app = loadImageInputApp();
+    const reason = 'Prompt uploads are not supported for remote (SSH) sessions';
+    app._uploadPasteImage = vi.fn(async () => {
+      throw new Error(reason);
+    });
+
+    await app._uploadAndInsertImages([{ path: '/tmp/shot.png' }]);
+
+    expect(app.showToast).toHaveBeenCalledWith(`1 failed: ${reason}`, 'error');
+  });
 });