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-lock.json

@@ -27,6 +27,7 @@
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       },
       "devDependencies": {
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         "@types/react": "~19.2.2",
         "typescript": "~6.0.3"
       }
@@ -2444,9 +2445,9 @@
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     },
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-      "resolved": "https://registry.npmjs.org/@types/node/-/node-26.2.0.tgz",
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+      "version": "26.4.1",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-26.4.1.tgz",
+      "integrity": "sha512-k97ENvZWtvA6yqz5/FS6a7duDgOPEeOQOc2iKS/nY6mX6qJUKtLnWzQS+Xj6tXweyj6ZcTAK2Qecetnvi9nCLA==",
       "license": "MIT",
       "dependencies": {
         "undici-types": "~8.3.0"