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>

src/lib/streak.ts

@@ -0,0 +1,56 @@
+// Pure streak logic over local calendar days — no React, no storage. Days are
+// "YYYY-MM-DD" keys in the device's local time zone, so a session at 23:50
+// counts for the day it was actually done, not the UTC one.
+
+export type DayKey = string;
+
+export function dayKey(date: Date): DayKey {
+  const y = date.getFullYear();
+  const m = String(date.getMonth() + 1).padStart(2, "0");
+  const d = String(date.getDate()).padStart(2, "0");
+  return `${y}-${m}-${d}`;
+}
+
+/** Local-midnight Date for a day key. */
+export function fromKey(key: DayKey): Date {
+  const [y, m, d] = key.split("-").map(Number);
+  return new Date(y, m - 1, d);
+}
+
+function shiftKey(key: DayKey, days: number): DayKey {
+  const date = fromKey(key);
+  date.setDate(date.getDate() + days);
+  return dayKey(date);
+}
+
+/** Unique, ascending day keys from a list of ISO session timestamps. */
+export function sessionDays(sessionsIso: string[]): DayKey[] {
+  return Array.from(new Set(sessionsIso.map((iso) => dayKey(new Date(iso))))).sort();
+}
+
+/** The last `n` day keys ending on `today`, oldest first. */
+export function lastNDays(n: number, today: DayKey): DayKey[] {
+  const out: DayKey[] = [];
+  for (let i = n - 1; i >= 0; i--) out.push(shiftKey(today, -i));
+  return out;
+}
+
+/**
+ * Current streak = consecutive trained days ending today or yesterday (a
+ * streak isn't broken until a full day is missed). Best = longest run ever.
+ */
+export function computeStreaks(days: DayKey[], today: DayKey): { current: number; best: number } {
+  const sorted = Array.from(new Set(days)).sort();
+  if (sorted.length === 0) return { current: 0, best: 0 };
+
+  let best = 1;
+  let run = 1;
+  for (let i = 1; i < sorted.length; i++) {
+    run = shiftKey(sorted[i - 1], 1) === sorted[i] ? run + 1 : 1;
+    if (run > best) best = run;
+  }
+
+  const last = sorted[sorted.length - 1];
+  const alive = last === today || last === shiftKey(today, -1);
+  return { current: alive ? run : 0, best };
+}