fix(windows): screen-reader accessibility for context menus and channel tree

- ContextMenuStrip: select first item on Opened so keyboard-invoked menus
  (Shift+F10 / Apps key) raise the UIA focus event immediately instead of
  staying silent until the first arrow key.
- Name the SplitContainer/SplitterPanel containers so screen readers announce
  orientation instead of a stack of anonymous pane nodes.
- Take the resize splitters out of the Tab cycle (TabStop = false) so focus
  moves control-to-control.
- RefreshChannelTree: restore keyboard focus and re-announce the current node
  after a Nodes.Clear()/rebuild, so the tree no longer loses focus when
  channels/users change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/windows/VoiceCat.App/Forms/MainForm.cs

@@ -201,6 +201,7 @@ public partial class MainForm : Form
                 ctx.Items.Add("&Delete channel...", null, (_, _) => DeleteSelectedChannel());
             }
         };
+        ctx.Opened += (_, _) => SelectFirstMenuItem(ctx);
         tvChannels.ContextMenuStrip = ctx;
     }
 
@@ -247,9 +248,26 @@ public partial class MainForm : Form
                 }
             }
         };
+        ctx.Opened += (_, _) => SelectFirstMenuItem(ctx);
         lstUsers.ContextMenuStrip = ctx;
     }
 
+    // Work around the WinForms ContextMenuStrip accessibility bug: when opened by
+    // keyboard (Shift+F10 / Apps key) the focused item is not set, so screen readers
+    // stay silent until the first arrow key. Selecting the first item ourselves on
+    // Opened raises the UIA focus event immediately.
+    private static void SelectFirstMenuItem(ContextMenuStrip ctx)
+    {
+        foreach (ToolStripItem item in ctx.Items)
+        {
+            if (item is ToolStripMenuItem && item.Enabled)
+            {
+                item.Select();
+                break;
+            }
+        }
+    }
+
     // ── Event dispatch ────────────────────────────────────────────────────────
 
     private void OnEvent(VoiceCatEvent ev)
@@ -468,6 +486,7 @@ public partial class MainForm : Form
     private void RefreshChannelTree()
     {
         uint toSelect = tvChannels.SelectedNode?.Tag is uint s ? s : _currentChannelId;
+        bool hadFocus = tvChannels.Focused;
         tvChannels.BeginUpdate();
         tvChannels.Nodes.Clear();
 
@@ -493,6 +512,18 @@ public partial class MainForm : Form
         tvChannels.ExpandAll();
         SeekAndSelect(tvChannels.Nodes, toSelect);
         tvChannels.EndUpdate();
+
+        // A Nodes.Clear()/rebuild can drop keyboard focus and leave the screen reader
+        // without a current node. If the tree was focused before the refresh, restore
+        // focus and re-announce the now-current node (null-then-reselect forces UIA to
+        // fire a fresh focus event).
+        if (hadFocus && tvChannels.SelectedNode != null)
+        {
+            var node = tvChannels.SelectedNode;
+            tvChannels.Focus();
+            tvChannels.SelectedNode = null;
+            tvChannels.SelectedNode = node;
+        }
     }
 
     private bool SeekAndSelect(TreeNodeCollection nodes, uint channelId)