Detect Umbrel ntfy and offer it as a built-in ntfy target

[schjonhaug]

Summary

Umbrel self-hosted Canary currently treats ntfy as fully manual: users type an ntfy server URL in Settings and optionally configure auth.

We should use the same Umbrel app-detection pattern as the explorer work so Canary can detect when Umbrel ntfy is installed and offer it as a built-in choice in Settings.

Current behavior

• Settings only expose a free-form ntfy server URL input today: frontend/src/components/settings/ntfy-server-settings.tsx
• User preferences only store a single ntfy_server_url plus auth fields: backend/src/auth.rs
• The backend publishes notifications server-side using the saved ntfy_server_url or falls back to NTFY_SERVER_URL / https://ntfy.sh
• The README already notes that on Umbrel the backend must use the Docker internal hostname, not the browser URL: README.md
• Umbrel ntfy is exposed behind the internal host ntfy_app_1: /Users/schjonhaug/Developer/canary-project/umbrel-apps/ntfy/docker-compose.yml

Requested support

When Umbrel ntfy is installed, Canary should detect it and let the user choose it in self-hosted Settings instead of making them manually type the internal Docker hostname.

Expected Umbrel ntfy configuration:

• ntfy URL: http://ntfy_app_1
• Auth: access token created in the user’s Umbrel ntfy instance

The resulting UX should make it easy to choose between:

• public https://ntfy.sh
• local Umbrel ntfy (http://ntfy_app_1)
• optionally, a manual custom URL for advanced users running some other ntfy instance

Product behavior

In self-hosted Umbrel settings:

• If Umbrel ntfy is not detected, keep current manual behavior
• If Umbrel ntfy is detected, show it as a selectable option
• Selecting Umbrel ntfy should use http://ntfy_app_1 as the effective backend target
• The user should still provide an ntfy access token from their Umbrel ntfy instance
• Test Notification should verify the selected ntfy target and token work end-to-end

Implementation notes

• Umbrel packaging:
  • Extend umbrel-apps/canary/exports.sh to detect installed app ntfy
  • Export metadata indicating Umbrel ntfy is available to Canary
• Backend:
  • Extend the self-hosted config/preferences contract to expose available ntfy options, not just a raw URL string
  • Preserve support for a manual custom URL
  • Keep cloud mode behavior unchanged
• Frontend:
  • Update the ntfy settings UI from raw-manual-only input to a simple choice UI with optional manual override
  • If Umbrel ntfy is selected, use http://ntfy_app_1 as the saved/effective server URL
  • Keep token configuration visible and straightforward, since Umbrel ntfy still requires an access token

Acceptance criteria

• Umbrel Canary detects when the Umbrel ntfy app is installed
• Self-hosted Settings exposes Umbrel ntfy as a built-in option when available
• Selecting Umbrel ntfy causes Canary backend publishes to use http://ntfy_app_1
• The user can still configure an ntfy access token for Umbrel ntfy
• Manual custom ntfy URL configuration remains possible
• Test Notification works with the selected ntfy target
• Existing non-Umbrel self-hosted ntfy configuration continues to work