feat(ui): rich AlarmTriggerEditor in instance override modal

Replaces the per-row JSON textbox with an Edit button that opens a modal
hosting the full AlarmTriggerEditor. The editor pre-populates with the
merged inherited + override config so the operator sees the effective
state, not the override delta.

On Save:
  - HiLo: diff against inherited, store only changed keys
  - Binary trigger types: whole-replace if the edited config differs

Value comparison in the diff is type-aware (decoded strings, numeric
GetDouble) so JSON-escape differences (e.g., literal em-dash vs —)
don't produce false-positive diffs that pollute the override JSON.

FlatteningService.MergeHiLoConfig is now public so the UI can pre-merge
the editor seed; new public DiffHiLoConfig handles the symmetric
direction. +2 encoding tests cover the new equivalence behavior.

The override row's summary column shows the diff'd keys + priority chip
so operators see what's overridden at a glance.

tests/ScadaLink.TemplateEngine.Tests/Flattening/FlatteningServiceMergeTests.cs

@@ -94,6 +94,100 @@ public class FlatteningServiceMergeTests
         Assert.Equal(3, doc.RootElement.GetProperty("hiDeadband").GetDouble());
     }
 
+    // ── DiffHiLoConfig ─────────────────────────────────────────────────────
+
+    [Fact]
+    public void DiffHiLoConfig_NoChanges_ReturnsNull()
+    {
+        const string both = @"{""attributeName"":""Temp"",""hi"":80}";
+        Assert.Null(FlatteningService.DiffHiLoConfig(both, both));
+    }
+
+    [Fact]
+    public void DiffHiLoConfig_ChangedKey_ReturnsOnlyChangedKey()
+    {
+        const string inherited = @"{""attributeName"":""Temp"",""loLo"":0,""lo"":10,""hi"":80,""hiHi"":100}";
+        const string edited = @"{""attributeName"":""Temp"",""loLo"":0,""lo"":10,""hi"":90,""hiHi"":100}";
+
+        var diff = FlatteningService.DiffHiLoConfig(inherited, edited);
+
+        Assert.NotNull(diff);
+        using var doc = JsonDocument.Parse(diff!);
+        var prop = Assert.Single(doc.RootElement.EnumerateObject());
+        Assert.Equal("hi", prop.Name);
+        Assert.Equal(90, prop.Value.GetDouble());
+    }
+
+    [Fact]
+    public void DiffHiLoConfig_NewKey_AddedToDiff()
+    {
+        const string inherited = @"{""attributeName"":""Temp"",""hi"":80}";
+        const string edited = @"{""attributeName"":""Temp"",""hi"":80,""hiDeadband"":3}";
+
+        var diff = FlatteningService.DiffHiLoConfig(inherited, edited);
+
+        Assert.NotNull(diff);
+        using var doc = JsonDocument.Parse(diff!);
+        Assert.Equal(3, doc.RootElement.GetProperty("hiDeadband").GetDouble());
+        Assert.False(doc.RootElement.TryGetProperty("hi", out _));
+    }
+
+    [Fact]
+    public void DiffHiLoConfig_NullInherited_ReturnsEditedVerbatim()
+    {
+        const string edited = @"{""attributeName"":""Temp"",""hi"":80}";
+        Assert.Equal(edited, FlatteningService.DiffHiLoConfig(null, edited));
+    }
+
+    [Fact]
+    public void DiffHiLoConfig_NullEdited_ReturnsNull()
+    {
+        Assert.Null(FlatteningService.DiffHiLoConfig(@"{""hi"":80}", null));
+    }
+
+    [Fact]
+    public void DiffHiLoConfig_IgnoresStringEscapeDifferences()
+    {
+        // Inherited has literal em-dash; edited has the unicode-escaped form.
+        // Decoded values are identical, so the key should NOT be in the diff.
+        const string inherited = @"{""attributeName"":""Temp"",""hi"":80,""hiMessage"":""High — investigate""}";
+        const string edited    = @"{""attributeName"":""Temp"",""hi"":80,""hiMessage"":""High — investigate""}";
+
+        var diff = FlatteningService.DiffHiLoConfig(inherited, edited);
+
+        Assert.Null(diff); // no real change once values are decoded
+    }
+
+    [Fact]
+    public void DiffHiLoConfig_IgnoresNumericFormatDifferences()
+    {
+        // 85 vs 85.0 are the same number — should not produce a diff.
+        const string inherited = @"{""hi"":85}";
+        const string edited    = @"{""hi"":85.0}";
+        Assert.Null(FlatteningService.DiffHiLoConfig(inherited, edited));
+    }
+
+    [Fact]
+    public void DiffHiLoConfig_RoundTripsThroughMerge()
+    {
+        // Merge(inherited, Diff(inherited, edited)) ≡ edited — when the
+        // edited config is itself a superset/equivalent of inherited.
+        const string inherited = @"{""attributeName"":""Temp"",""hi"":80,""hiHi"":100}";
+        const string edited = @"{""attributeName"":""Temp"",""hi"":90,""hiHi"":100,""hiDeadband"":5}";
+
+        var diff = FlatteningService.DiffHiLoConfig(inherited, edited);
+        var merged = FlatteningService.MergeHiLoConfig(inherited, diff);
+
+        using var origDoc = JsonDocument.Parse(edited);
+        using var mergedDoc = JsonDocument.Parse(merged!);
+        Assert.Equal(origDoc.RootElement.GetProperty("hi").GetDouble(),
+                     mergedDoc.RootElement.GetProperty("hi").GetDouble());
+        Assert.Equal(origDoc.RootElement.GetProperty("hiHi").GetDouble(),
+                     mergedDoc.RootElement.GetProperty("hiHi").GetDouble());
+        Assert.Equal(origDoc.RootElement.GetProperty("hiDeadband").GetDouble(),
+                     mergedDoc.RootElement.GetProperty("hiDeadband").GetDouble());
+    }
+
     // ── Instance-level alarm override (end-to-end Flatten) ─────────────────
 
     private static (Template, Instance) BuildHiLoFixture(string inheritedJson, InstanceAlarmOverride? ovr = null, bool locked = false)