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.Designer.cs

@@ -140,6 +140,13 @@ partial class MainForm
         splitLeft.Panel1MinSize = 100;
         splitLeft.Panel2MinSize = 80;
         splitLeft.TabIndex = 0;
+        // Keep the resize splitter out of the Tab cycle so focus moves control-to-control.
+        splitLeft.TabStop = false;
+        // Name the otherwise-anonymous "pane" containers so screen readers announce
+        // orientation instead of a stack of unnamed panes.
+        splitLeft.AccessibleName = "Channels and users";
+        splitLeft.Panel1.AccessibleName = "Channels";
+        splitLeft.Panel2.AccessibleName = "Users";
         splitLeft.Panel1.Controls.Add(tvChannels);
         splitLeft.Panel1.Controls.Add(lblChannels);
         splitLeft.Panel2.Controls.Add(lstUsers);
@@ -214,6 +221,10 @@ partial class MainForm
         splitMain.Dock = DockStyle.Fill;
         splitMain.Panel1MinSize = 150;
         splitMain.TabIndex = 1;
+        splitMain.TabStop = false;
+        splitMain.AccessibleName = "Main";
+        splitMain.Panel1.AccessibleName = "Channels and users";
+        splitMain.Panel2.AccessibleName = "Chat and activity";
         splitMain.Panel1.Controls.Add(splitLeft);
         splitMain.Panel2.Controls.Add(tblRight);