feat(figma): design dark mode billing and subscription interfaces

[rexx010]

Summary

Designed the Dark Mode UI/UX for StellarProof's billing, subscription, and transaction fee experiences in Figma, covering the requested interfaces and maintaining consistency with the platform's existing design system.

What Was Done

• Created high-fidelity Dark Mode designs for the four requested interfaces.
• Designed billing dashboard and invoice table views for clear data presentation and plan-limit visibility.
• Designed subscription management flows for upgrading, downgrading, and cancelling plans.
• Designed the Gas Fee Estimator UI to clearly communicate estimated Stellar network fees before confirmation.
• Applied consistent Dark Mode surfaces, typography, spacing, alignment, and contrast.
• Used reusable Figma components and auto-layout for maintainable designs.
• Created responsive layouts and variants where appropriate.
• Organized frames, layers, and components with clear naming and structure.
• Ensured visual consistency across the billing and subscription experience.

Scope

This PR contains Figma/UI/UX design work only and does not include frontend implementation.

Related Issues

Closes #577
Closes #578
Closes #579
Closes #580

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