Share subscription stats calculation

[er1c-cartman]

Summary

• Extract shared subscription stats calculation into src/utils/stats.ts
• Reuse the shared calculation from dummyData.getTotalMonthlySpending() and subscriptionStore.calculateStats()
• Add focused unit coverage for empty input, mixed billing cycles, inactive subscriptions, category counts, gas totals, and injected currency conversion
• Restore the malformed subscriptionStore state object so the file parses before the stats refactor runs

Validation

• PATH=/home/goalie/.local/node-v22/bin:$PATH npx --yes tsx -e "...stats smoke..." -> stats smoke passed
• PATH=/home/goalie/.local/node-v22/bin:$PATH npx --yes prettier --check src/utils/stats.ts src/utils/__tests__/stats.test.ts src/utils/dummyData.ts src/store/subscriptionStore.ts
• git diff --check

Dependency note

• npm ci currently fails on upstream dependency metadata before tests can run locally: first with the ethers peer conflict, then with No matching version found for @react-native/jest-preset@^0.1.0 when retried with --legacy-peer-deps.

Closes #63