feat(ui): show GPG signed badge on signed commits in graph view

Signed commits now display a '✓ Signed' badge in the commit graph
next to the message. The badge uses the Success color (green) for
visibility. Commit signing status is already tracked in CommitInfo
(is_signed) but was not previously rendered in the graph UI.

Author: Forge

crates/rgitui_graph/src/lib.rs

@@ -1687,6 +1687,15 @@ impl Render for GraphView {
                                 }
                             }
 
+                            // GPG signed commit badge
+                            if commit.is_signed {
+                                message_col = message_col.child(
+                                    div()
+                                        .flex_shrink_0()
+                                        .child(Badge::new("✓ Signed").color(Color::Success).bold()),
+                                );
+                            }
+
                             let summary_label = if is_head_row {
                                 Label::new(summary)
                                     .size(LabelSize::Small)