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).

src/ScadaLink.CentralUI/Components/Shared/DurationInput.cs

@@ -0,0 +1,50 @@
+using System.Globalization;
+
+namespace ScadaLink.CentralUI.Components.Shared;
+
+/// <summary>
+/// Converts a <see cref="TimeSpan"/> to and from the number+unit pair behind a
+/// duration input (milliseconds / seconds / minutes). A blank or non-positive
+/// number represents "unset" (a <c>null</c> duration).
+/// </summary>
+internal static class DurationInput
+{
+    /// <summary>The unit tokens a duration input offers, smallest first.</summary>
+    internal static readonly string[] Units = { "ms", "sec", "min" };
+
+    /// <summary>
+    /// Splits a duration into the largest whole unit that represents it exactly.
+    /// A null or non-positive duration yields a blank value and the default
+    /// <c>sec</c> unit.
+    /// </summary>
+    internal static (string? Value, string Unit) Split(TimeSpan? duration)
+    {
+        if (duration is not { } d || d <= TimeSpan.Zero) return (null, "sec");
+
+        var ms = (long)d.TotalMilliseconds;
+        if (ms % 60000 == 0) return ((ms / 60000).ToString(CultureInfo.InvariantCulture), "min");
+        if (ms % 1000 == 0) return ((ms / 1000).ToString(CultureInfo.InvariantCulture), "sec");
+        return (ms.ToString(CultureInfo.InvariantCulture), "ms");
+    }
+
+    /// <summary>
+    /// Composes a number+unit pair into a duration. A blank, unparseable, or
+    /// non-positive value yields <c>null</c> (unset).
+    /// </summary>
+    internal static TimeSpan? Compose(string? value, string unit)
+    {
+        if (!long.TryParse(value, NumberStyles.Integer, CultureInfo.InvariantCulture, out var n)
+            || n <= 0)
+        {
+            return null;
+        }
+
+        var factorMs = unit switch
+        {
+            "min" => 60000L,
+            "ms" => 1L,
+            _ => 1000L,
+        };
+        return TimeSpan.FromMilliseconds(n * factorMs);
+    }
+}

src/ScadaLink.CentralUI/Components/Shared/ScriptTriggerConfigCodec.cs

@@ -81,6 +81,17 @@ internal static class ScriptTriggerConfigCodec
         };
     }
 
+    /// <summary>
+    /// Whether a trigger type honours the script's <c>MinTimeBetweenRuns</c>.
+    /// True for the auto-firing triggers it throttles — ValueChange, Conditional,
+    /// Expression. False for Interval (its own period is the cadence), Call
+    /// (invoked explicitly, never throttled), and None/Unknown.
+    /// </summary>
+    internal static bool SupportsMinTimeBetweenRuns(string? triggerType) =>
+        ParseKind(triggerType) is ScriptTriggerKind.ValueChange
+            or ScriptTriggerKind.Conditional
+            or ScriptTriggerKind.Expression;
+
     /// <summary>Canonical <c>TriggerType</c> string for a kind; null for None/Unknown.</summary>
     internal static string? KindToString(ScriptTriggerKind kind) => kind switch
     {