fix: stabilize send scan routing

[ovitrif]

Description

Fixes review blockers for #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 inside SendSheet before routing.

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

Preview

Before	After

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.