fix: graceful null handling and idempotent stop

[ovitrif]

Summary

• Make LightningService.stop() idempotent (succeed if node is already null instead of throwing)
• Pass node reference to listenForEvents to use a stable reference throughout the event loop
• Handle node being stopped gracefully in event listener (exit loop instead of crashing)
• Handle null balances/channels gracefully in getBalancesAsync/getChannelsAsync
• Always cancel scope in LightningNodeService.onDestroy() since stop is now idempotent

Context

This addresses pre-existing issues exposed by the crash logs shared in PR #717:

1. getBalancesAsync crash: checkNotNull(lightningService.balances) threw IllegalStateException='Required value was null.' because node?.listBalances() returned null when the underlying LDK node was stopped
2. Non-idempotent stop: LightningService.stop() threw NodeNotStarted when node was already null
3. Event listener instability: listenForEvents accessed this.node on each loop iteration, which could return null mid-loop when stop was called concurrently

Note: The node property is already @Volatile which provides sufficient thread-safety for the reference itself. These are not thread-safety issues with the reference - they're about handling the case when the LDK node object is stopped and its methods return null.

Test plan

• Build succeeds: ./gradlew compileDevDebugKotlin
• Unit tests pass: ./gradlew testDevDebugUnitTest
• Lint clean: ./gradlew detekt
• Manual test: verify the crashes reported on parent PR Handle node connection issues #717 dont repro

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.