feat(ui): chat Copy mode — select + Ctrl+C any text in the chat tail

User reported wanting to mark text in-game and copy it out (item names,
coordinates, NPC dialogue, etc). ImGui doesn't natively let you select
across multiple TextColored widgets, but a read-only multi-line
InputText is fully click-drag selectable + Ctrl+C copyable. This
commit adds a "Copy mode" toggle to ChatPanel that swaps the chat
tail's render path between the colored-line view and a single
selectable text region.

New IPanelRenderer primitive:

  void TextMultilineReadOnly(string id, string content, Vector2 size);

ImGui maps this to InputTextMultiline with the ReadOnly flag — same
selection + Ctrl+C UX a user expects from any text-input widget.
FakePanelRenderer records the call for tests. The future D.2b
custom retail-look backend implements its own equivalent (likely
the same widget pattern with retail font/skin).

ChatPanel rendering:

  · A "Copy mode (select text to Ctrl+C)" Checkbox at the top of
    the panel toggles _copyMode.
  · Off (default) — current per-line render with colored combat
    entries. Visually unchanged from before.
  · On — the chat tail becomes a single TextMultilineReadOnly
    widget holding every visible line joined with newlines. Loses
    per-line color, gains arbitrary-span text selection.
  · Footer (separator + input field) renders identically in both
    modes so the user can still type while in copy mode.

Existing ChatPanelLayoutTests's footer-separator probe was using
IndexOf("Separator") — which now matches the new pre-tail separator
between the Checkbox and the chat tail. Switched to LastIndexOf
which still pins the footer separator (between EndChild and
InputTextSubmit). Behaviour and intent unchanged.

DisplaySettingsTests' With_expression test was still asserting the
old "1920x1080" Default.Resolution; updated to the new "1280x720"
that the previous wire-up commit introduced (the earlier commit
forgot this one).

dotnet build green (0 warnings); dotnet test 1,309 / 1,309 green
(243 Core.Net + 393 UI.Abstractions + 673 Core).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.UI.ImGui/ImGuiPanelRenderer.cs

@@ -207,4 +207,21 @@ public sealed class ImGuiPanelRenderer : IPanelRenderer
 
     /// <inheritdoc />
     public void EndTabItem() => ImGuiNET.ImGui.EndTabItem();
+
+    // -- Selectable / copyable text ---------------------------------------
+
+    /// <inheritdoc />
+    public void TextMultilineReadOnly(string id, string content, Vector2 size)
+    {
+        // ImGui's InputTextMultiline takes a `ref string` even with the
+        // ReadOnly flag — we just hand it a local copy. maxLength caps
+        // what the user could type if ReadOnly were ever cleared; we
+        // size it to the current content (+1 for ImGui's internal NUL
+        // terminator in some bindings). Min of 1 keeps the empty case
+        // from confusing native bindings.
+        string buffer = content;
+        uint maxLen = (uint)System.Math.Max(content.Length + 1, 1);
+        ImGuiNET.ImGui.InputTextMultiline(id, ref buffer, maxLen, size,
+            ImGuiInputTextFlags.ReadOnly);
+    }
 }