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/ui/mod.rs

@@ -1,4 +1,5 @@
 pub mod app;
+pub mod cat_tree;
 pub mod category_panel;
 pub mod effect;
 pub mod formula_panel;