Payment URI handling: add desktop startup, single-instance, and file-open routing

[johnny9]

Code Gap

The QML wallet has Bitcoin URI parsing support in the Send flow, but it does not yet have the Qt widgets PaymentServer style desktop integration.

Missing pieces include startup command-line URI capture, routing from a second application instance into the running app, macOS file/open event handling, and app-level delivery of valid bitcoin: URIs to the active Send flow.

Work Needed

• Add desktop/app integration for bitcoin: URI open requests.
• Capture payment URIs passed at startup.
• Route URI requests from an already-running instance where supported.
• Handle platform file/open events where applicable.
• Validate and report malformed URI requests using QML app-shell UI.
• Deliver valid requests into the Send flow without losing wallet context.

Priority

v1.0 / P2 secondary enhancement unless promoted for desktop integration parity.