feat: build subscription pause and resume with billing adjustment

[senmalong]

Summary

Implements production-ready subscription pause and resume functionality with automatic billing schedule adjustment across Soroban smart contracts (contracts/subscription/src/) and the Zustand frontend store (src/store/subscriptionStore.ts).

Changes Implemented

• Smart Contracts (contracts/subscription/src/):
  • proration.rs: Added PauseAdjustmentResult struct and calculate_pause_adjustment helper function to compute billing adjustments.
  • lib.rs: Updated check_and_resume_internal and resume_subscription to shift next_charge_at forward by the actual elapsed pause duration (min(now - paused_at, pause_duration)). Added preview_pause_adjustment for frontend/SDK previews.
• Frontend State Store (src/store/ & src/types/):
  • src/types/subscription.ts: Added pause state properties (isPaused, pausedAt, pauseDurationDays, pausedUntil, billingAdjustmentAmount) to the Subscription interface.
  • src/store/subscriptionStore.ts: Implemented pauseSubscription, resumeSubscription, and previewPauseAdjustment store actions with calendar/reminder synchronization and stats calculation.
• Documentation:
  • Added docs/SUBSCRIPTION_PAUSE_RESUME.md detailing contract state transitions, formulas, and usage examples.

Closes #961

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[Smartdevs17]

Thanks for contributing! The changes have been merged. Feel free to leave a review or feedback.