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/cat_tree.rs

@@ -0,0 +1,51 @@
+use crate::model::Model;
+use std::collections::HashSet;
+
+/// A flattened entry in the category panel tree.
+#[derive(Debug, Clone)]
+pub enum CatTreeEntry {
+    /// Category header row: name, item count, expanded?
+    Category {
+        name: String,
+        item_count: usize,
+        expanded: bool,
+    },
+    /// Item row under a category
+    Item { cat_name: String, item_name: String },
+}
+
+impl CatTreeEntry {
+    /// The category this entry belongs to.
+    pub fn cat_name(&self) -> &str {
+        match self {
+            CatTreeEntry::Category { name, .. } => name,
+            CatTreeEntry::Item { cat_name, .. } => cat_name,
+        }
+    }
+}
+
+/// Build the flattened tree of categories and their items.
+pub fn build_cat_tree(model: &Model, expanded: &HashSet<String>) -> Vec<CatTreeEntry> {
+    let mut entries = Vec::new();
+    for cat_name in model.category_names() {
+        let cat = model.category(cat_name);
+        let item_count = cat.map(|c| c.items.len()).unwrap_or(0);
+        let is_expanded = expanded.contains(cat_name);
+        entries.push(CatTreeEntry::Category {
+            name: cat_name.to_string(),
+            item_count,
+            expanded: is_expanded,
+        });
+        if is_expanded {
+            if let Some(cat) = cat {
+                for item_name in cat.ordered_item_names() {
+                    entries.push(CatTreeEntry::Item {
+                        cat_name: cat_name.to_string(),
+                        item_name: item_name.to_string(),
+                    });
+                }
+            }
+        }
+    }
+    entries
+}