feat(ui): replace loading spinners with shimmer skeleton screens

[fantababa]

Closes #67

Replace ActivityIndicator and blank loading states with smooth shimmer
skeleton screens to improve perceived performance on initial data load.

• SkeletonLoader: rewrote with a linear shimmer animation using
  expo-linear-gradient and Animated translateX on the native driver,
  replacing the previous opacity pulse. Added StatsCardSkeleton and
  SubscriptionDetailSkeleton to match the real layouts exactly
• HomeScreen: reads isLoading from useSubscriptionStore and renders
  StatsCardSkeleton + SubscriptionListSkeleton (4 cards) while data
  is fetching; FilterBar and header remain visible during load
• SubscriptionDetailScreen: replaces ActivityIndicator with
  SubscriptionDetailSkeleton wrapped in SafeAreaView
• All skeleton components use useThemeColors() so shimmer colors
  adapt to light and dark mode automatically

Pull Request Checklist

Quality Gates (All must pass before merge)

• Lint: Code passes ESLint and Prettier checks
• Type Check: TypeScript compilation succeeds
• Tests: All tests pass
• Build: Project builds successfully
• Rust Format: Smart contract formatting is correct
• Rust Clippy: Smart contract linting passes
• Rust Tests: All smart contract tests pass
• Rust Build: Smart contracts compile successfully

Additional Requirements

• New code has appropriate TypeScript types
• No hardcoded secrets or credentials
• New features have corresponding tests
• Documentation updated if needed

Reviewers

• At least 1 approval required for merge
• All CI checks must be green

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This PR will not be mergeable until all quality gates pass.

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