feat(chat): enhance chat UI with new features and visual improvements

- Added new attributes for chat colors including rail, date rules, and unread markers.
- Enhanced ChatLine class to support colored segments for continuation lines and horizontal separator rules.
- Introduced a WelcomeBanner class to display a splash screen when no channel is selected.
- Implemented NickColorHelper for deterministic per-nick colors, ensuring consistent user color representation.
- Updated ChannelListSource to highlight channels with unread messages and mentions.
- Improved MainWindow to include a spinner for transitional connection states and display activity in the status bar.
- Added unit tests for NickColorHelper to ensure stable and predictable color indexing.

src/EchoHub.Tests/NickColorHelperTests.cs

@@ -0,0 +1,55 @@
+using EchoHub.Client.UI.Helpers;
+using Xunit;
+
+namespace EchoHub.Tests;
+
+/// <summary>
+/// Tests for the deterministic nick→palette-index hash.
+/// Note: GetAttribute (Terminal.Gui Attribute) is excluded — Terminal.Gui's module
+/// initializer requires a display driver unavailable in CI.
+/// </summary>
+public class NickColorHelperTests
+{
+    [Fact]
+    public void GetPaletteIndex_SameNick_IsStable()
+    {
+        var first = NickColorHelper.GetPaletteIndex("alice", 12);
+        var second = NickColorHelper.GetPaletteIndex("alice", 12);
+        Assert.Equal(first, second);
+    }
+
+    [Fact]
+    public void GetPaletteIndex_IsCaseInsensitive()
+    {
+        Assert.Equal(
+            NickColorHelper.GetPaletteIndex("Alice", 12),
+            NickColorHelper.GetPaletteIndex("aLICE", 12));
+    }
+
+    [Theory]
+    [InlineData("alice")]
+    [InlineData("bob")]
+    [InlineData("charlie_long_nickname")]
+    [InlineData("")]
+    [InlineData("émile")]
+    public void GetPaletteIndex_AlwaysWithinRange(string nick)
+    {
+        var index = NickColorHelper.GetPaletteIndex(nick, 12);
+        Assert.InRange(index, 0, 11);
+    }
+
+    [Fact]
+    public void GetPaletteIndex_DistributesAcrossPalette()
+    {
+        // Not a strict uniformity test — just that the hash isn't degenerate
+        var nicks = new[] { "alice", "bob", "carol", "dave", "erin", "frank", "grace", "heidi" };
+        var distinct = nicks.Select(n => NickColorHelper.GetPaletteIndex(n, 12)).Distinct().Count();
+        Assert.True(distinct >= 3, $"Expected at least 3 distinct palette slots, got {distinct}");
+    }
+
+    [Fact]
+    public void GetPaletteIndex_NonPositivePaletteSize_ReturnsZero()
+    {
+        Assert.Equal(0, NickColorHelper.GetPaletteIndex("alice", 0));
+    }
+}