[REQ-001] Add currentStreak and bestStreak pure functions with unit tests

[markoub]

User story

As a user who tracks habits, I want the app to compute my current and best streaks so that I can see my momentum at a glance.

Acceptance criteria

• currentStreak(habit: Habit, today: string): number is added to src/habits.ts.
  • Returns the count of consecutive completed days ending on today.
  • If today is not completed but yesterday (today − 1 day) is, the streak still counts up through yesterday.
  • Returns 0 when there is no active streak (no completion on today or yesterday).
• bestStreak(habit: Habit): number is added to src/habits.ts.
  • Returns the length of the longest run of consecutive completed days ever recorded.
  • Returns 0 for a habit with no completions.
• Both functions are pure — they accept today as a YYYY-MM-DD string parameter; they never read Date, localStorage, or the DOM.
• Unit tests in src/habits.test.ts cover all of the following cases for each function:
  • Empty habit (no completions) → 0.
  • Single completed day (today) → 1.
  • Single completed day (yesterday only, today not done) → currentStreak returns 1; bestStreak returns 1.
  • Streak broken in the middle → correct count before and after the gap.
  • Active streak of several days ending today.
  • Best streak is a past run, not the current run.
• npm test and npm run build both pass with no new TypeScript errors.

Definition of done

npm test and npm run build pass; all new logic lives in src/habits.ts (pure, no DOM/localStorage/window); every new branch is covered by a test in src/habits.test.ts.

Source: REQ-001 (vault/02 - Requirements/REQ-001 - Streaks and momentum.md)

[markoub]

Implementation is ready for review: #3

PR adds currentStreak and bestStreak to src/habits.ts (pure functions, no DOM/storage) with 12 new unit tests. All 17 tests pass and the build is green.

---

Generated by Claude Code