fix: stabilize send scan routing

[ovitrif]

Description

Fixes issues surfaced in #878:

• Avoids waiting 15 seconds for usable channels when there are no channels or the node cannot become runnable.
• Stabilizes scan routing by preserving whether a scan started from the main scanner or send sheet before routing.
• Adds a changelog fragment for the user-facing fix.

Also fixes all lint issues accumulated in latest PRs (should later update build config to fail on lint errors).

Preview

Before	After
before.mp4	after.mp4

QA Notes

Manual Tests

• 1. No usable channels/spending balance → scan LN invoice: error shows immediately, not after 15s.
• 2. regression: Scanner → scan fixed amount LN invoice: Send Confirm or QuickPay opens directly.
• 3a. regression: Send → scanner/paste fixed amount LN invoice: in-sheet nav to Confirm or QuickPay.
  • 3b. regression: Variable amount LN invoice/LNURL-pay: lands on Amount view.

Automated Tests

Added new unit tests:

• main scanner lightning scan opens send sheet
• main scanner zero amount lightning scan opens amount sheet
• main scanner lightning scan opens QuickPay when enabled
• waitForUsableChannels waits for running state before treating empty channels as absent

[ovitrif]

Addressed the changelog fragment review note in bf5e8f06a by adding changelog.d/next/925.fixed.md and updating the PR description to remove the stale no-changelog note.

[claude]

CLAUDE.md violation: prefer Duration for timeouts and delays

CHANNELS_USABLE_TIMEOUT_MS is a raw Long constant (15_000L), but the CLAUDE.md rule states: "ALWAYS prefer Kotlin Duration for timeouts and delays".

This PR already introduces NO_USABLE_CHANNELS_FEEDBACK_DELAY = 2_500.milliseconds using Duration correctly, but reuses the old raw Long constant in both new call sites (here and line 1052).

The fix is to convert the constant to Duration:

// In companion object — change from:
private const val CHANNELS_USABLE_TIMEOUT_MS = 15_000L
// to:
private val CHANNELS_USABLE_TIMEOUT = 15.seconds

Then update both call sites: withTimeoutOrNull(CHANNELS_USABLE_TIMEOUT_MS) → withTimeoutOrNull(CHANNELS_USABLE_TIMEOUT). Note: MS_SYNC_LOOP_DEBOUNCE and SYNC_RETRY_DELAY_MS are pre-existing and not touched by this PR.

[claude]

Code review

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