feat: day streaks and two-week activity history

Session count alone says nothing about consistency, which is what makes
pelvic floor training work. Record every completion timestamp and derive
streaks from local calendar days: current streak (survives until a full
day is missed), best streak, and a 14-day dot strip on the home screen.
The done screen now shows the streak so finishing is rewarded.

- src/lib/streak.ts: pure day-key / streak logic, covered by node --test
- store: new `sessions` array, defaulted for pre-existing installs
- home: StreakCard, screen now scrolls for smaller phones
- tsconfig: TS 6 no longer auto-includes @types, so declare node types

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -15,13 +15,15 @@ A session is four guided blocks driven by a step timer with haptic cues:
 3. **Quick squeezes** — 10 × 1s burst contractions with 1s release.
 4. **Quick pushes** — 10 × 1s burst pushes with 1s release.
 
-Progress (session count, difficulty adjustment) lives in AsyncStorage — no
-server, no account.
+Progress (session count, difficulty adjustment, completion history) lives in
+AsyncStorage — no server, no account. The home screen shows your **day streak**
+(alive until a full day is missed), best streak, and a two-week activity strip.
 
 ## Layout
 
 - `src/app/` — expo-router screens: `index` (home), `session` (timer), `settings`
 - `src/lib/workout.ts` — pure session-plan + progression logic
+- `src/lib/streak.ts` — pure streak / calendar-day logic (tested)
 - `src/lib/store.ts` — tiny local store persisted to AsyncStorage
 - `android/` — committed prebuild output; CI builds it directly
 
@@ -31,6 +33,7 @@ server, no account.
 npm install
 npm start            # Expo dev server
 npm run typecheck
+npm test             # node --test over src/lib/*.test.ts
 ```
 
 ## Release