fix(attendance): timezone-proof print dates; overnight-only time prefixes

Two bugs in the attendance admin print (Tisk on Správa docházky):

1. Viewer-timezone date shift: day rows are built from date-only strings
   ('2026-06-01'), which new Date() parses as UTC midnight per spec; local
   formatting then rendered the PREVIOUS day for any viewer west of UTC —
   a June print opened with '31. 5. Neděle' (a May row holding June 1's
   punches) and every weekday name + weekend tint shifted by one. Czech
   viewers were unaffected (UTC+1/+2), US viewers always hit it.
   formatDate / getCzechWeekday / isWeekendDate now parse the calendar day
   from the string parts — identical output in every timezone.

2. Every time cell carried a bogus date prefix ('1.6. 08:00'): the
   overnight-only guard compared the extracted date part against the raw
   wire shift_date ('2026-06-01T02:00:00') — never equal. Now compares
   normalized date parts; the prefix fires only for true overnight punches.

Verified end-to-end against the production June payload; +8 regression
tests (timezone-proof by construction — parts-based, no UTC parse).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/__tests__/attendance-print-tz.test.ts

@@ -0,0 +1,77 @@
+import { describe, it, expect } from "vitest";
+import {
+  getCzechWeekday,
+  isWeekendDate,
+  formatTimeOrDatetimePrint,
+} from "../admin/utils/attendanceHelpers";
+import { formatDate } from "../admin/utils/formatters";
+
+/**
+ * Regression: the attendance admin print builds its day rows from date-ONLY
+ * strings ("2026-06-01"). `new Date("YYYY-MM-DD")` is UTC midnight per the
+ * ECMAScript spec, so formatting it with local getters rendered the PREVIOUS
+ * day for viewers west of UTC — a June print opened with "31. 5. 2026 Neděle"
+ * (a May row) and every weekday name was shifted by one. The helpers now
+ * parse the calendar day from the string parts, which is timezone-proof by
+ * construction: these assertions hold in EVERY zone (on a machine west of
+ * UTC the old implementation fails them).
+ */
+describe("attendance print — timezone-proof date rendering", () => {
+  it("formatDate renders a date-only string as that exact calendar day", () => {
+    expect(formatDate("2026-06-01")).toBe("1. 6. 2026");
+    expect(formatDate("2026-06-30")).toBe("30. 6. 2026");
+  });
+
+  it("formatDate keeps naive-datetime (wire format) behavior", () => {
+    expect(formatDate("2026-06-01T02:00:00")).toBe("1. 6. 2026");
+  });
+
+  it("getCzechWeekday names the string's own calendar day", () => {
+    expect(getCzechWeekday("2026-06-01")).toBe("Pondělí");
+    expect(getCzechWeekday("2026-06-06")).toBe("Sobota");
+    expect(getCzechWeekday("2026-06-07")).toBe("Neděle");
+    // naive-datetime wire format resolves to the same day
+    expect(getCzechWeekday("2026-06-01T02:00:00")).toBe("Pondělí");
+  });
+
+  it("isWeekendDate flags the string's own calendar day", () => {
+    expect(isWeekendDate("2026-06-06")).toBe(true); // Saturday
+    expect(isWeekendDate("2026-06-07")).toBe(true); // Sunday
+    expect(isWeekendDate("2026-06-01")).toBe(false); // Monday
+    expect(isWeekendDate("2026-06-06T02:00:00")).toBe(true);
+  });
+
+  it("agrees with a locally-constructed date in the current zone (guards a regression to UTC parsing)", () => {
+    // Local construction is the ground truth for "calendar day" in any zone.
+    const local = new Date(2026, 5, 1, 12).toLocaleDateString("cs-CZ", {
+      weekday: "long",
+    });
+    const expected = local.charAt(0).toUpperCase() + local.slice(1);
+    expect(getCzechWeekday("2026-06-01")).toBe(expected);
+  });
+});
+
+describe("attendance print — overnight date prefix on time cells", () => {
+  // shift_date arrives as a naive datetime on the wire ("2026-06-01T02:00:00");
+  // the guard used to compare it raw against the extracted date part, so the
+  // overnight-only "d.m." prefix fired on EVERY cell ("1.6. 08:00").
+  const wireShiftDate = "2026-06-01T02:00:00";
+
+  it("same-day punch renders time only", () => {
+    expect(
+      formatTimeOrDatetimePrint("2026-06-01T08:00:00", wireShiftDate),
+    ).toBe("08:00");
+  });
+
+  it("overnight punch keeps the date prefix", () => {
+    expect(
+      formatTimeOrDatetimePrint("2026-06-02T01:30:00", wireShiftDate),
+    ).toBe("2.6. 01:30");
+  });
+
+  it("date-only shiftDate also matches", () => {
+    expect(formatTimeOrDatetimePrint("2026-06-01T08:00:00", "2026-06-01")).toBe(
+      "08:00",
+    );
+  });
+});