Congratulatory message on increment

[SaketRane]

When /api/streak/increment returns an increment (vs no-op), emit a client event to show a toast/modal: “Nice! Day N streak 🔥”.
Add milestone variants at N ∈ {1, 3, 7, 30} with unique copy (e.g., badge preview).
Persist a one-shot flag per day (UserStreakToast { userId, dateShown }) so it doesn’t re-show on page reloads.

Analytics: streak_increment, streak_milestone_shown, streak_toast_closed.

Acceptance criteria: After completing a module, user sees a single congrats message (once per day); milestone copy shows at the right counts; events fire with { userId, day }.