continuation of (#257)

[L03TJ3]

• fix: improve G$ calculator text readability
• feat: improve Buy G$ widget with event handling fixes and responsive design
• wip: add url signing flow
• include server-side url-signing flow
• feat: Added a storybook for the converter styling

Summary by Sourcery

Implement secure, responsive improvements to the Buy G$ (Onramper) flow and enhance the G$ converter readability and documentation.

New Features:

• Add server-side URL signing flow for the Onramper widget via backend integration.
• Introduce responsive sizing and loading state handling for the embedded Onramper WebView widget across breakpoints.
• Add Storybook stories for the Onramper Buy G$ widget configuration and the G$ converter calculator, including interactive examples.

Bug Fixes:

• Fix swap flow lock so it resets correctly on errors to prevent stuck state in the Buy G$ process.
• Improve event parsing for Onramper messages to handle multiple event field formats and avoid failures on non-JSON payloads.

Enhancements:

• Refine Onramper widget UX with better error feedback, wallet validation messaging, and progress event tracking for the stepper.
• Optimize AsyncStorage access and URL construction in the Onramper widget for performance and clearer logging.
• Improve G$ converter styling for better text hierarchy, contrast, and readability in the calculator UI.

Documentation:

• Document features and behavior of the Onramper widget and G$ converter via enriched Storybook stories, including styling and event-flow notes.