feat: add category tree with expand/collapse in category panel

Add a tree-based category panel that supports expand/collapse of categories.

Introduces CatTreeEntry and build_cat_tree to render categories as
a collapsible tree. The category panel now displays categories with
expand indicators (▶/▼) and shows items under expanded categories.

CmdContext gains cat_tree_entry(), cat_at_cursor(), and cat_tree_len()
methods to work with the tree. App tracks expanded_cats in a HashSet.

Keymap updates: Enter in category panel now triggers filter-to-item.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/Qwen3.5-35B-A3B-GGUF:Q5_K_M)

src/draw.rs

@@ -228,7 +228,12 @@ fn draw_content(f: &mut Frame, area: Rect, app: &App) {
         if app.category_panel_open {
             let a = Rect::new(side.x, y, side.width, ph);
             f.render_widget(
-                CategoryPanel::new(&app.model, &app.mode, app.cat_panel_cursor),
+                CategoryPanel::new(
+                    &app.model,
+                    &app.mode,
+                    app.cat_panel_cursor,
+                    &app.expanded_cats,
+                ),
                 a,
             );
             y += ph;