feat(settings): clock format setting (Auto / 24h / 12h) (#742)

* feat(settings): clock format setting (Auto / 24h / 12h)

Reported on the Psysonic Discord — the Queue side panel's ETA label
and the sleep-timer preview both render via `formatClockTime`, which
just calls `toLocaleTimeString` and so follows the user's system
locale. On en-US that means AM/PM, with no in-app way out.

Add a tri-state **Clock Format** setting under
**Settings → System → App Behavior**:

* `auto` (default) — keep the existing locale-driven behaviour, so
  bestehende installs are unaffected on first launch.
* `24h` — force 24-hour wall-clock output everywhere
  `formatClockTime` is used.
* `12h` — force AM/PM output.

Wired through `authStore` (`clockFormat`, `setClockFormat`), exposed
via `CustomSelect` in `SystemTab`, and threaded into the two
consumers (`QueueHeader`, `PlaybackDelayModal`) so they re-render on
change. `formatClockTime` itself stays a pure helper — it accepts the
setting as an optional second argument and maps it to `hour12`.

Locale coverage: all nine bundled locales (en, de, es, fr, nl, nb,
ru, zh, ro) get the four new settings strings. Pin tests added for
the `setClockFormat` setter and the `hour12` mapping in
`formatClockTime`.

* docs(changelog): clock format setting + contributors (PR #742)

src/utils/format/formatClockTime.test.ts

@@ -0,0 +1,32 @@
+/**
+ * `formatClockTime` is a thin wrapper around `Date.toLocaleTimeString` whose
+ * only knob is `hour12`, mapped from the user's `ClockFormat` setting. The
+ * exact `HH:MM` output is locale-dependent and not asserted here — these
+ * tests pin only the `hour12` mapping, which is what the setting controls.
+ */
+import { describe, expect, it } from 'vitest';
+import { formatClockTime } from './formatClockTime';
+
+const SAMPLE_TS = Date.UTC(2026, 0, 1, 19, 17, 0); // 19:17 UTC, deterministic
+
+describe('formatClockTime — clockFormat mapping', () => {
+  it('forces 24-hour output when clockFormat === "24h" (no AM/PM marker)', () => {
+    const out = formatClockTime(SAMPLE_TS, '24h');
+    expect(out).not.toMatch(/AM|PM/i);
+  });
+
+  it('forces 12-hour output when clockFormat === "12h" (renders an AM/PM marker)', () => {
+    const out = formatClockTime(SAMPLE_TS, '12h');
+    expect(out).toMatch(/AM|PM/i);
+  });
+
+  it('falls through to the locale default when clockFormat === "auto"', () => {
+    // We do not assert AM/PM either way here — `'auto'` deliberately defers to
+    // the JS engine's locale. The contract is just "do not force `hour12`".
+    expect(() => formatClockTime(SAMPLE_TS, 'auto')).not.toThrow();
+  });
+
+  it('falls through to the locale default when clockFormat is omitted', () => {
+    expect(() => formatClockTime(SAMPLE_TS)).not.toThrow();
+  });
+});