feat: add view history navigation and drill-into-cell

Add view navigation history with back/forward stacks (bound to < and >).

Introduce CategoryKind enum to distinguish regular categories from
virtual ones (_Index, _Dim) that are synthesized at query time.

Add DrillIntoCell command that creates a drill view showing raw data
for an aggregated cell, expanding categories on Axis::None into Row
and Column axes while filtering by the cell's fixed coordinates.

Virtual categories default to Axis::None and are automatically added
to all views when the model is initialized.

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

src/ui/effect.rs

@@ -112,10 +112,41 @@ impl Effect for DeleteView {
 pub struct SwitchView(pub String);
 impl Effect for SwitchView {
     fn apply(&self, app: &mut App) {
+        let current = app.model.active_view.clone();
+        if current != self.0 {
+            app.view_back_stack.push(current);
+            app.view_forward_stack.clear();
+        }
         let _ = app.model.switch_view(&self.0);
     }
 }
 
+/// Go back in view history (pop back stack, push current to forward stack).
+#[derive(Debug)]
+pub struct ViewBack;
+impl Effect for ViewBack {
+    fn apply(&self, app: &mut App) {
+        if let Some(prev) = app.view_back_stack.pop() {
+            let current = app.model.active_view.clone();
+            app.view_forward_stack.push(current);
+            let _ = app.model.switch_view(&prev);
+        }
+    }
+}
+
+/// Go forward in view history (pop forward stack, push current to back stack).
+#[derive(Debug)]
+pub struct ViewForward;
+impl Effect for ViewForward {
+    fn apply(&self, app: &mut App) {
+        if let Some(next) = app.view_forward_stack.pop() {
+            let current = app.model.active_view.clone();
+            app.view_back_stack.push(current);
+            let _ = app.model.switch_view(&next);
+        }
+    }
+}
+
 #[derive(Debug)]
 pub struct SetAxis {
     pub category: String,