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/cmd.rs

@@ -47,10 +47,35 @@ pub struct CmdContext<'a> {
     /// The display value at the cursor in records mode (including any
     /// pending edit override). None for normal pivot views.
     pub records_value: Option<String>,
+    /// How many data rows/cols fit on screen (for viewport scrolling).
+    /// Defaults to generous fallbacks when unknown.
+    pub visible_rows: usize,
+    pub visible_cols: usize,
+    /// Expanded categories in the tree panel
+    pub expanded_cats: &'a std::collections::HashSet<String>,
     /// The key that triggered this command
     pub key_code: KeyCode,
 }
 
+impl<'a> CmdContext<'a> {
+    /// Resolve the category panel tree entry at the current cursor.
+    pub fn cat_tree_entry(&self) -> Option<crate::ui::cat_tree::CatTreeEntry> {
+        let tree = crate::ui::cat_tree::build_cat_tree(self.model, self.expanded_cats);
+        tree.into_iter().nth(self.cat_panel_cursor)
+    }
+
+    /// The category name at the current tree cursor (whether on a
+    /// category header or an item).
+    pub fn cat_at_cursor(&self) -> Option<String> {
+        self.cat_tree_entry().map(|e| e.cat_name().to_string())
+    }
+
+    /// Total number of entries in the category tree.
+    pub fn cat_tree_len(&self) -> usize {
+        crate::ui::cat_tree::build_cat_tree(self.model, self.expanded_cats).len()
+    }
+}
+
 /// A command that reads state and produces effects.
 pub trait Cmd: Debug + Send + Sync {
     fn execute(&self, ctx: &CmdContext) -> Vec<Box<dyn Effect>>;
@@ -217,6 +242,8 @@ pub struct CursorState {
     pub col_count: usize,
     pub row_offset: usize,
     pub col_offset: usize,
+    pub visible_rows: usize,
+    pub visible_cols: usize,
 }
 
 impl CursorState {
@@ -228,6 +255,8 @@ impl CursorState {
             col_count: ctx.col_count,
             row_offset: ctx.row_offset,
             col_offset: ctx.col_offset,
+            visible_rows: ctx.visible_rows,
+            visible_cols: ctx.visible_cols,
         }
     }
 }