fix(central-ui): resolve CentralUI-007..014 — nav authz, UTC date filters, disposal guards, N+1 fix, async script analysis

src/ScadaLink.CentralUI/Components/BrowserTime.cs

@@ -0,0 +1,41 @@
+namespace ScadaLink.CentralUI.Components;
+
+/// <summary>
+/// Converts <c>&lt;input type="datetime-local"&gt;</c> values — which are always
+/// expressed in the user's <i>browser-local</i> time zone — into UTC
+/// <see cref="DateTimeOffset"/>s for querying.
+/// <para>
+/// CLAUDE.md mandates UTC throughout the system, but a <c>datetime-local</c>
+/// value carries no offset, so it must be <i>converted</i> to UTC, not relabelled
+/// as UTC. Relabelling (the CentralUI-008 bug) shifts every query window by the
+/// user's offset for any non-UTC browser.
+/// </para>
+/// </summary>
+public static class BrowserTime
+{
+    /// <summary>
+    /// Converts a browser-local <paramref name="localValue"/> to UTC using the
+    /// browser's <c>Date.getTimezoneOffset()</c> result.
+    /// </summary>
+    /// <param name="localValue">
+    /// The wall-clock value from a <c>datetime-local</c> input, or <c>null</c>.
+    /// </param>
+    /// <param name="browserUtcOffsetMinutes">
+    /// The value of JavaScript <c>new Date().getTimezoneOffset()</c>: the number
+    /// of minutes that, <b>added</b> to local time, yields UTC. It is positive
+    /// for time zones behind UTC (e.g. +300 for UTC-5) and negative for zones
+    /// ahead (e.g. -120 for UTC+2).
+    /// </param>
+    /// <returns>The equivalent instant in UTC, or <c>null</c> when the input is null.</returns>
+    public static DateTimeOffset? LocalInputToUtc(DateTime? localValue, int browserUtcOffsetMinutes)
+    {
+        if (localValue is not { } local)
+            return null;
+
+        // getTimezoneOffset() is defined as (UTC - local) in minutes, so
+        // UTC = local + offset.
+        var utc = DateTime.SpecifyKind(local, DateTimeKind.Unspecified)
+            .AddMinutes(browserUtcOffsetMinutes);
+        return new DateTimeOffset(utc, TimeSpan.Zero);
+    }
+}