Add unit tests for the bill urgency engine in lib/bills/urgency.ts

[Skinny001]

This pull request adds comprehensive unit tests for the bill urgency utilities and makes minor improvements to code consistency and test configuration. The new tests ensure correct behavior for date calculations, urgency status, and day formatting, while the config changes guarantee these tests are picked up by the test runner.

Testing improvements:

• Added a new test file lib/bills/urgency.test.ts with thorough unit tests for daysDiff, computeUrgency, and computeDaysInfo, covering edge cases like leap years, time zones, and overdue/upcoming logic.

Configuration updates:

• Updated vitest.config.mts to include all test files under lib/ for test discovery and to exclude them from type checking, ensuring the new tests are run and not type-checked redundantly. [1] [2]

Code consistency:

• Standardized string quoting style to double quotes in lib/bills/urgency.ts for all urgency-related strings. [1] [2]

close #469