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>

package.json

@@ -22,6 +22,7 @@
     "react-native-web": "^0.21.2"
   },
   "devDependencies": {
+    "@types/node": "^26.4.1",
     "@types/react": "~19.2.2",
     "typescript": "~6.0.3"
   },
@@ -29,7 +30,8 @@
     "start": "expo start",
     "android": "expo run:android",
     "ios": "expo run:ios",
-    "typecheck": "tsc --noEmit"
+    "typecheck": "tsc --noEmit",
+    "test": "node --test src/lib/*.test.ts"
   },
   "private": true
 }