feat: add pending payment notifications

[ovitrif]

This PR:

1. Adds push notifications for pending payment resolution when the app is backgrounded
2. Extracts PendingPaymentRepo from LightningRepo to manage pending payment state independently
3. Simplifies SendPendingViewModel by using resolution events directly from the repo
4. Fixes non-service notification icon to use the outlined Bitkit icon

Description

When a Lightning payment is pending (e.g. hold invoices) and the user backgrounds the app, payment resolution events (success/failure) were silently ignored. This adds a CQRS command+handler (NotifyPendingPaymentResolved) that checks whether a resolved payment was pending, and if the app is in the background, posts a system notification.

The pending payment tracking logic has been extracted from LightningRepo into a dedicated PendingPaymentRepo with proper StateFlow-based state management. The SendPendingViewModel and QuickPayViewModel now consume resolution events directly without mapping to intermediate types.

Also:

• Removes the error message from the sheet screen for pending payment error.
• Fixes the notification icon for non-service notifications.
• Added unit tests for business logic.

Preview

bgnotifs_1.75x.mp4

Note

The regression issue with the error toast showing for the same cancelled pending payment handled by the sheet, as seen in the recording has been fixed in 27a378b

QA Notes

Setup

1. Startup bitkit-docker: docker compose up -d
2. Start emulator via Android Studio
3. Apply port forwarding:

   adb reverse tcp:9735 tcp:9735 && \
   adb reverse tcp:3000 tcp:3000 && \
   adb reverse tcp:60001 tcp:60001

4. Apply local-electrum.patch to connect to local Electrum, then build and run on emulator:

   curl -sL https://gist.githubusercontent.com/ovitrif/ac13a7ba09efd4c12bee0a1b4cc27f4a/raw/local-electrum.patch | git apply

5. Open a channel, skip first 4 steps

Use ./bitcoin-cli in bitkit-docker for hold invoices:

./bitcoin-cli holdinvoice 42 -m 'test'
./bitcoin-cli settleinvoice __preimage__
./bitcoin-cli cancelinvoice __hash__

Tests

• App in BG + settle/cancel → success/error notification
• Regression Checks
  • Pending sheet + settle/cancel → success/error screen
  • Close button + settle/cancel → success/error toast
  • Scanner (QuickPay on/off), paste, cancel invoice, tap 'Try Again' → expect Send Recipient screen
  • Send a regular Lightning payment (non-hold invoice)