choret: enforce discord-only provider type across notifications API and UI

- Added validation to reject non-discord provider types in create, update, test, and preview operations.
- Updated the notifications form to automatically normalize non-discord types to discord.
- Modified UI to display explicit messaging for deprecated and non-dispatch statuses for non-discord providers.
- Enhanced tests to cover new validation logic and UI changes for provider types.

frontend/src/api/notifications.test.ts

@@ -88,6 +88,16 @@ describe('notifications api', () => {
     expect(mockedClient.delete).toHaveBeenCalledWith('/notifications/providers/new')
   })
 
+  it('rejects non-discord type before submit for provider mutations and preview', async () => {
+    await expect(createProvider({ name: 'Bad', type: 'slack' })).rejects.toThrow('Only discord notification providers are supported')
+    await expect(updateProvider('bad', { type: 'generic' })).rejects.toThrow('Only discord notification providers are supported')
+    await expect(testProvider({ id: 'bad', type: 'email' })).rejects.toThrow('Only discord notification providers are supported')
+    await expect(previewProvider({ id: 'bad', type: 'gotify' })).rejects.toThrow('Only discord notification providers are supported')
+
+    expect(mockedClient.post).not.toHaveBeenCalled()
+    expect(mockedClient.put).not.toHaveBeenCalled()
+  })
+
   it('fetches templates and previews provider payloads with data', async () => {
     mockedClient.get.mockResolvedValueOnce({ data: [{ id: 'tpl', name: 'default' }] })
     mockedClient.post.mockResolvedValue({ data: { preview: 'ok' } })