Fix: Log-Zeitstempel in lokaler Zeit (mit Offset) statt UTC

User-Report: Logs zeigten z.B. "17:29:43" obwohl es lokal 19:29:43 war (CEST/UTC+2), weil
alle Logger `new Date().toISOString()` (UTC "...Z") nutzten. Neuer Helper logTimestamp()
formatiert lokale Zeit mit explizitem Offset (ISO 8601, z.B. "2026-05-31T19:29:43.605+02:00")
— menschlich lokal UND weiterhin eindeutig/Date.parse-bar. Angewandt auf alle Log-Zeilen-
Writer: item-log, logger (rd_downloader.log), audit-log, rename-log, session-log,
package-log, account-rotation-log, trace-log. Interne/API-/Dateinamen-Zeitstempel
(debug-server, support-bundle, trace autoDisableAt-Config) bleiben absichtlich UTC.

Test: tests/log-timestamp.test.ts (Format + Round-Trip zum selben Instant + lokale Stunde,
TZ-unabhaengig). 650 Tests gruen, tsc 9, Build sauber.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/log-timestamp.test.ts

@@ -0,0 +1,28 @@
+import { describe, expect, it } from "vitest";
+import { logTimestamp } from "../src/main/log-timestamp";
+
+describe("logTimestamp", () => {
+  it("formats local time with an explicit UTC offset (ISO 8601), not a UTC 'Z' string", () => {
+    const instant = new Date("2026-05-31T17:29:43.605Z");
+    const formatted = logTimestamp(instant);
+
+    // Shape: YYYY-MM-DDTHH:MM:SS.mmm±HH:MM
+    expect(formatted).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}[+-]\d{2}:\d{2}$/);
+    // The whole point: NOT the old UTC "...Z" format that showed 17:29 instead of 19:29.
+    expect(formatted.endsWith("Z")).toBe(false);
+  });
+
+  it("is parseable back to the exact same instant (offset keeps it unambiguous)", () => {
+    const instant = new Date("2026-05-31T17:29:43.605Z");
+    // Date.parse must still recover the identical instant (trace-log autoDisableAt etc.).
+    expect(new Date(logTimestamp(instant)).getTime()).toBe(instant.getTime());
+  });
+
+  it("shows the LOCAL wall-clock hour (machine-timezone-independent assertion)", () => {
+    const instant = new Date("2026-05-31T17:29:43.605Z");
+    const formatted = logTimestamp(instant);
+    // Hour segment must equal the local getHours() of the same instant — i.e. the
+    // user's wall clock, whatever the server timezone is.
+    expect(formatted.slice(11, 13)).toBe(String(instant.getHours()).padStart(2, "0"));
+  });
+});