test: add tests for string error display and double-click prevention

Tauri invoke errors are strings (not Error instances). The new test
verifies the frontend displays the actual backend error message.
Also tests that the login button is disabled during the OAuth flow.

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

src/components/SettingsPanel.test.tsx

@@ -291,5 +291,56 @@ describe('SettingsPanel', () => {
       )
       expect(screen.getByText(/Connect your GitHub account/)).toBeInTheDocument()
     })
+
+    it('displays the actual backend error string when device flow start fails', async () => {
+      mockInvokeFn.mockImplementation(async (cmd: string) => {
+        if (cmd === 'github_device_flow_start') {
+          // Tauri invoke rejects with a plain string, not an Error instance
+          throw 'GitHub device flow not available. Ensure a GitHub App is registered.'
+        }
+        return null
+      })
+
+      render(
+        <SettingsPanel open={true} settings={emptySettings} onSave={onSave} onClose={onClose} />
+      )
+
+      fireEvent.click(screen.getByTestId('github-login'))
+
+      await waitFor(() => {
+        expect(screen.getByTestId('github-error')).toHaveTextContent(
+          'GitHub device flow not available. Ensure a GitHub App is registered.'
+        )
+      })
+    })
+
+    it('prevents double-click by disabling button during login flow', async () => {
+      let resolveStart: ((v: unknown) => void) | null = null
+      mockInvokeFn.mockImplementation(async (cmd: string) => {
+        if (cmd === 'github_device_flow_start') {
+          return new Promise(r => { resolveStart = r })
+        }
+        return null
+      })
+
+      render(
+        <SettingsPanel open={true} settings={emptySettings} onSave={onSave} onClose={onClose} />
+      )
+
+      const loginBtn = screen.getByTestId('github-login') as HTMLButtonElement
+      fireEvent.click(loginBtn)
+
+      // Button should be disabled while waiting
+      await waitFor(() => {
+        expect(loginBtn.disabled).toBe(true)
+      })
+
+      // Clean up
+      resolveStart?.({
+        device_code: 'dc', user_code: 'UC-1234',
+        verification_uri: 'https://github.com/login/device',
+        expires_in: 900, interval: 5,
+      })
+    })
   })
 })