fix(due,store,ui): month grid steps by calendar day so dst cannot repeat one

tests/due.test.ts

@@ -1,8 +1,10 @@
-import { describe, expect, it } from "vitest";
+import { afterAll, beforeAll, describe, expect, it } from "vitest";
 import {
   daysFromToday,
   dueLabel,
   dueTone,
+  localKey,
+  monthGrid,
   nextFriday,
 } from "../src/lib/due";
 
@@ -26,3 +28,30 @@ describe("due helpers read the day they are given", () => {
     expect(nextFriday("2026-08-28")).toBe("2026-09-04");
   });
 });
+
+describe("monthGrid steps by calendar day", () => {
+  const zone = process.env.TZ;
+  beforeAll(() => {
+    process.env.TZ = "Europe/Berlin";
+  });
+  afterAll(() => {
+    process.env.TZ = zone;
+  });
+
+  it("keeps every day once across the autumn clock change", () => {
+    const keys = monthGrid(new Date(2026, 9, 1)).map(localKey);
+    expect(keys).toHaveLength(42);
+    expect(new Set(keys).size).toBe(42);
+    expect(keys[0]).toBe("2026-09-28");
+    expect(keys[27]).toBe("2026-10-25");
+    expect(keys[28]).toBe("2026-10-26");
+    expect(keys[41]).toBe("2026-11-08");
+  });
+
+  it("keeps every day once across the spring clock change", () => {
+    const keys = monthGrid(new Date(2027, 2, 1)).map(localKey);
+    expect(new Set(keys).size).toBe(42);
+    expect(keys).toContain("2027-03-28");
+    expect(keys).toContain("2027-03-29");
+  });
+});