fix(glp1): file timeline side effects under the mood entry's own day

The timeline grouped side effects by the UTC day of moodLoggedAt, so an
evening entry west of UTC (or a morning one east of it) showed on the
neighbouring day. The entry's stored date is already the writer's local
day; group by that.

src/app/api/insights/glp1-timeline/__tests__/route.test.ts

@@ -54,6 +54,7 @@ function medication() {
 
 function moodDay(day: number, tags: string[]) {
   return {
+    date: `2026-04-${String(day).padStart(2, "0")}`,
     moodLoggedAt: new Date(Date.UTC(2026, 3, day, 9, 0, 0)),
     tags: JSON.stringify(tags),
   };
@@ -99,6 +100,20 @@ describe("GET /api/insights/glp1-timeline side-effect days", () => {
     ]);
   });
 
+  it("files a side effect under the entry's own day, not its UTC day", async () => {
+    // Logged 2026-04-10 at 20:00 in Los Angeles: already the 11th in UTC.
+    const out = await timeline([
+      {
+        date: "2026-04-10",
+        moodLoggedAt: new Date("2026-04-11T03:00:00.000Z"),
+        tags: JSON.stringify(["nausea"]),
+      },
+    ]);
+    expect(out.entries).toEqual([
+      { date: "2026-04-10T12:00:00Z", kind: "side-effect", tags: ["nausea"] },
+    ]);
+  });
+
   it("recognises the chip label of every shipped locale", async () => {
     const out = await timeline([
       moodDay(10, ["Übelkeit"]),

src/app/api/insights/glp1-timeline/route.ts

@@ -80,7 +80,7 @@ export const GET = apiHandler(async (request: NextRequest) => {
       deletedAt: null,
       moodLoggedAt: { gte: ninetyDaysAgo },
     },
-    select: { moodLoggedAt: true, tags: true },
+    select: { date: true, tags: true },
     orderBy: { moodLoggedAt: "desc" },
   });
 
@@ -140,7 +140,10 @@ export const GET = apiHandler(async (request: NextRequest) => {
     const { matched, unresolvedCount } = matchGlp1SideEffectTags(mood.tags);
     unresolvedTagCount += unresolvedCount;
     if (matched.length === 0) continue;
-    const dayKey = mood.moodLoggedAt.toISOString().slice(0, 10);
+    // `date` is the day the entry belongs to in the writer's zone. The UTC
+    // day of `moodLoggedAt` moved an evening entry west of UTC (a morning
+    // one east of it) onto the neighbouring day.
+    const dayKey = mood.date;
     const existing = sideEffectByDay.get(dayKey) ?? [];
     for (const t of matched) {
       if (!existing.includes(t)) existing.push(t);