fix(charts): end the mood and intake heatmaps on the user's today

Both heatmaps stepped back from now in 24-hour strides and cut each key in
UTC, so in the evening west of UTC, or the morning east of it, the newest
cell was a different day from the one the server keyed the readings on. The
days now come from today's key in the user's display zone, stepped on keys.

src/lib/charts/__tests__/heatmap-days.test.ts

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+import { describe, expect, it } from "vitest";
+
+import { heatmapDays } from "../heatmap-days";
+
+describe("heatmapDays", () => {
+  it.each([
+    // 20:30 in New York is already the next UTC day.
+    ["America/New_York", "2026-03-09T00:30:00.000Z", "2026-03-08"],
+    // 08:00 in Auckland and Tonga is still the previous UTC day.
+    ["Pacific/Auckland", "2026-04-04T20:00:00.000Z", "2026-04-05"],
+    ["Pacific/Tongatapu", "2026-07-27T19:00:00.000Z", "2026-07-28"],
+    ["America/Santiago", "2026-09-06T03:30:00.000Z", "2026-09-05"],
+    ["Europe/Berlin", "2026-10-25T22:30:00.000Z", "2026-10-25"],
+  ])("ends on today's key in %s", (tz, iso, today) => {
+    const days = heatmapDays(new Date(iso), tz, 14);
+    expect(days).toHaveLength(14);
+    expect(days.at(-1)?.dateKey).toBe(today);
+    // Consecutive keys, one per day, no gap or repeat across DST.
+    for (let i = 1; i < days.length; i++) {
+      const prev = Date.parse(`${days[i - 1].dateKey}T00:00:00Z`);
+      const cur = Date.parse(`${days[i].dateKey}T00:00:00Z`);
+      expect(cur - prev).toBe(86_400_000);
+    }
+  });
+
+  it("reads weekday and month off the key", () => {
+    const [last] = heatmapDays(
+      new Date("2026-03-01T03:00:00.000Z"),
+      "America/New_York",
+      1,
+    );
+    // 22:00 on Saturday 28 February in New York.
+    expect(last).toEqual({ dateKey: "2026-02-28", dow: 5, month: 1 });
+  });
+});

src/lib/charts/heatmap-days.ts

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+import { shiftDateKey, userDayKey } from "@/lib/tz/format";
+
+/** One calendar-heatmap column entry: a local day and where it sits. */
+export interface HeatmapDay {
+  /** `YYYY-MM-DD` in the user's zone. */
+  dateKey: string;
+  /** Weekday with Monday = 0 … Sunday = 6. */
+  dow: number;
+  /** Month index 0–11. */
+  month: number;
+}
+
+/**
+ * The last `days` local days in `timeZone`, oldest first, ending with today.
+ *
+ * Built from today's key in the user's zone and stepped on keys. The heatmaps
+ * used to take `now − n·24 h` and cut the key in UTC, so in the evening west
+ * of UTC (or the morning east of it) "today" was a different day from the one
+ * the server had keyed the user's readings on, and the newest cell showed the
+ * wrong day. Weekday and month come from the key itself.
+ */
+export function heatmapDays(
+  now: Date,
+  timeZone: string,
+  days: number,
+): HeatmapDay[] {
+  const today = userDayKey(now, timeZone);
+  const out: HeatmapDay[] = [];
+  for (let back = days - 1; back >= 0; back--) {
+    const dateKey = shiftDateKey(today, -back);
+    const [y, m, d] = dateKey.split("-").map(Number);
+    const utc = new Date(Date.UTC(y, m - 1, d));
+    out.push({ dateKey, dow: (utc.getUTCDay() + 6) % 7, month: m - 1 });
+  }
+  return out;
+}