Implement subscription credit system and account balance

[chinedu15]

Closes #947

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Updated PR Checklist

• Lint: Focused ESLint with --no-ignore passed; Prettier passed.
• Type Check: Full TypeScript check still fails on unrelated existing files:
  • alerting.ts
  • CancellationFlowScreen.tsx
  • SupportDashboardScreen.tsx
  • fraud.ts
• Credit Tests: 13 JavaScript unit/integration tests passed.
• All Tests: Only the focused credit tests were run, not the entire repository suite.
• Build: Still fails with Expo/Metro error: ./src/lib/TerminalReporter is not exported.
• Rust Format: Passed.
• Rust Clippy: Passed.
• Rust Tests: 11 passed.
• Rust Build: Release build passed.
• Coverage: 95.17% statements, 85.71% branches, 97.5% functions, 99.2% lines.
• Performance: Benchmark passed in 88 ms, under the 1,500 ms budget.
• Documentation: Updated.
• Branch synchronization: Branch matches origin.

Closes #947

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