feat(central-ui): add Min time between runs field to the script form

The template script editor had no input for MinTimeBetweenRuns, so a
WhileTrue trigger configured through the UI always saved a null interval
and degraded to a single edge fire. The Add/Edit Script modal now has a
"Min time between runs" number+unit (ms/sec/min) field.

- Visible only for ValueChange / Conditional / Expression triggers — the
  auto-firing triggers MinTimeBetweenRuns throttles. Hidden for Interval
  (its own period is the cadence), Call (invoked explicitly, never
  throttled), and None.
- For a WhileTrue Conditional/Expression trigger the field is labelled as
  the re-fire interval and shows a warning while it is blank.
- Wired through the new-script and edit-script save paths (edit previously
  only preserved the existing value, never let the user change it).

New DurationInput helper does the TimeSpan <-> number+unit conversion;
ScriptTriggerConfigCodec.SupportsMinTimeBetweenRuns classifies trigger
types. Both TDD'd — 21 new tests. CentralUI suite 316 green; verified
end-to-end in the browser (visibility per trigger type, WhileTrue warning,
save/reload round-trip).

tests/ScadaLink.CentralUI.Tests/Shared/DurationInputTests.cs

@@ -0,0 +1,98 @@
+using ScadaLink.CentralUI.Components.Shared;
+
+namespace ScadaLink.CentralUI.Tests.Shared;
+
+/// <summary>
+/// Coverage for <see cref="DurationInput"/>, the number+unit codec behind the
+/// script form's "Min time between runs" field.
+/// </summary>
+public class DurationInputTests
+{
+    // ── Split: TimeSpan -> (value, unit) ───────────────────────────────────
+
+    [Fact]
+    public void Split_Null_ReturnsBlankWithSecondsUnit()
+    {
+        var (value, unit) = DurationInput.Split(null);
+
+        Assert.Null(value);
+        Assert.Equal("sec", unit);
+    }
+
+    [Fact]
+    public void Split_Zero_ReturnsBlank()
+    {
+        var (value, _) = DurationInput.Split(TimeSpan.Zero);
+
+        Assert.Null(value);
+    }
+
+    [Fact]
+    public void Split_WholeMinutes_UsesMinuteUnit()
+    {
+        var (value, unit) = DurationInput.Split(TimeSpan.FromMinutes(5));
+
+        Assert.Equal("5", value);
+        Assert.Equal("min", unit);
+    }
+
+    [Fact]
+    public void Split_WholeSeconds_UsesSecondUnit()
+    {
+        var (value, unit) = DurationInput.Split(TimeSpan.FromSeconds(30));
+
+        Assert.Equal("30", value);
+        Assert.Equal("sec", unit);
+    }
+
+    [Fact]
+    public void Split_SubSecond_UsesMillisecondUnit()
+    {
+        var (value, unit) = DurationInput.Split(TimeSpan.FromMilliseconds(250));
+
+        Assert.Equal("250", value);
+        Assert.Equal("ms", unit);
+    }
+
+    // ── Compose: (value, unit) -> TimeSpan? ────────────────────────────────
+
+    [Fact]
+    public void Compose_Blank_ReturnsNull() =>
+        Assert.Null(DurationInput.Compose(null, "sec"));
+
+    [Fact]
+    public void Compose_Zero_ReturnsNull() =>
+        Assert.Null(DurationInput.Compose("0", "sec"));
+
+    [Fact]
+    public void Compose_Negative_ReturnsNull() =>
+        Assert.Null(DurationInput.Compose("-5", "sec"));
+
+    [Fact]
+    public void Compose_SecondsValue_BuildsDuration() =>
+        Assert.Equal(TimeSpan.FromSeconds(30), DurationInput.Compose("30", "sec"));
+
+    [Fact]
+    public void Compose_MinutesValue_BuildsDuration() =>
+        Assert.Equal(TimeSpan.FromMinutes(5), DurationInput.Compose("5", "min"));
+
+    [Fact]
+    public void Compose_MillisecondsValue_BuildsDuration() =>
+        Assert.Equal(TimeSpan.FromMilliseconds(250), DurationInput.Compose("250", "ms"));
+
+    // ── Round-trip ─────────────────────────────────────────────────────────
+
+    [Theory]
+    [InlineData(250)]
+    [InlineData(30000)]
+    [InlineData(300000)]
+    public void RoundTrip_PreservesDuration(long milliseconds)
+    {
+        var original = TimeSpan.FromMilliseconds(milliseconds);
+
+        var (value, unit) = DurationInput.Split(original);
+        var reparsed = DurationInput.Compose(value, unit);
+
+        Assert.Equal(original, reparsed);
+    }
+}