refactor(ui): simplify panel mode mapping and add scroll tests

Adds a `mode()` method to the `Panel` enum to map panels to their
corresponding `AppMode`. Simplifies `TogglePanelAndFocus` in `cmd.rs`
to use this method instead of a manual match block.

Also adds regression tests in `app.rs` to verify that viewport
scrolling now correctly handles small terminal heights.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-31B-it-GGUF:UD-Q5_K_XL)

src/ui/effect.rs

@@ -830,6 +830,16 @@ pub enum Panel {
     View,
 }
 
+impl Panel {
+    pub fn mode(self) -> AppMode {
+        match self {
+            Panel::Formula => AppMode::FormulaPanel,
+            Panel::Category => AppMode::CategoryPanel,
+            Panel::View => AppMode::ViewPanel,
+        }
+    }
+}
+
 impl Effect for SetPanelOpen {
     fn apply(&self, app: &mut App) {
         match self.panel {