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/command/keymap.rs

@@ -449,6 +449,27 @@ impl KeymapSet {
         );
         cp.bind(KeyCode::Char('a'), none, "open-item-add-at-cursor");
         cp.bind(KeyCode::Char('o'), none, "open-item-add-at-cursor");
+        cp.bind(KeyCode::Char('d'), none, "delete-category-at-cursor");
+        cp.bind(KeyCode::Delete, none, "delete-category-at-cursor");
+        // C/F/V in panel modes: close panel (toggle-panel-and-focus sees focused=true)
+        cp.bind_args(
+            KeyCode::Char('C'),
+            none,
+            "toggle-panel-and-focus",
+            vec!["category".into()],
+        );
+        cp.bind_args(
+            KeyCode::Char('F'),
+            none,
+            "toggle-panel-and-focus",
+            vec!["formula".into()],
+        );
+        cp.bind_args(
+            KeyCode::Char('V'),
+            none,
+            "toggle-panel-and-focus",
+            vec!["view".into()],
+        );
         set.insert(ModeKey::CategoryPanel, Arc::new(cp));
 
         // ── View panel ───────────────────────────────────────────────────