[REQ-001] Show current streak count and 🔥 badge per habit in the UI

[markoub]

User story

As a user, I want to see my current streak number next to each habit, and a 🔥 badge when I'm on a roll (3+ days), so that I feel rewarded for consistency.

Acceptance criteria

• Each habit row in src/main.ts displays the current streak count returned by currentStreak(habit, today).
• When the current streak is ≥ 3, a 🔥 emoji badge is shown alongside the streak count.
• When the current streak is 0, no streak number or badge is displayed (clean / uncluttered).
• today is obtained from the existing dayStr() helper — never hardcoded.
• The streak display updates immediately when the user marks a habit done or undone (i.e. re-renders correctly on each toggle).
• No new runtime dependencies are introduced.
• npm test and npm run build both pass.

Definition of done

npm test and npm run build pass; streak display is wired to currentStreak from src/habits.ts; all DOM/rendering changes are confined to src/main.ts.

Depends on: the issue that adds currentStreak to src/habits.ts ([REQ-001] Add currentStreak and bestStreak pure functions with unit tests).

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

[markoub]

Implementation is ready for review: #4

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