test: add unit tests, ESLint, Prettier, Husky, and CI pipeline

[audemed44]

Summary

• Server refactoring: Routes converted to factory pattern (createApp(db), createXxxRouter(db)) to enable in-memory SQLite testing without touching the production DB
• 74 server tests via Vitest + supertest — 99% line coverage across all routes, DB mappers, recurring processor, and logger
• 119 client tests via Vitest + Testing Library — 96% line coverage across API client, finance utilities, CSV import/export, hooks, and mobile detection
• ESLint (flat config with typescript-eslint + react-hooks + react-refresh) and Prettier for consistent code style
• Husky pre-commit hook: runs lint-staged (auto-fix ESLint + Prettier on staged files) then full test suite before every commit
• commitlint enforces conventional commit format on every commit message
• CI pipeline updated with 3 parallel jobs: lint, test (with coverage artifact upload), build

Coverage thresholds (both workspaces)

Metric	Threshold	Server actual	Client actual
Lines	90%	99.1%	96.4%
Statements	80%	98.6%	95.6%
Branches	80%	88.6%	85.1%
Functions	80%	97.0%	90.6%

Test plan

• npm run lint — 0 errors
• npm run test:coverage — all 193 tests pass, all thresholds met
• npm run build — server + client build successfully
• Commitlint rejects bad message (bad commit message → fails)
• Commitlint accepts valid message (feat(auth): add login → passes)
• Pre-commit hook fires lint-staged + tests on git commit

🤖 Generated with Claude Code