feat(ui): simplify AppMode minibuffer handling and panel rendering

Refactor AppMode to use MinibufferConfig for all text-entry modes. Update
command implementations to use new mode constructors. Introduce
PanelContent trait and replace panel structs with content types. Adjust
rendering to use Panel::new and minibuffer configuration. Update imports
and add MinibufferConfig struct. No functional changes; all behavior
preserved.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (bartowski/nvidia_Nemotron-Cascade-2-30B-A3B-GGUF)

src/ui/effect.rs

@@ -108,9 +108,7 @@ impl Effect for EnterEditAtCursor {
         let value = ctx.display_value.clone();
         drop(ctx);
         app.buffers.insert("edit".to_string(), value);
-        app.mode = AppMode::Editing {
-            buffer: String::new(),
-        };
+        app.mode = AppMode::editing();
     }
 }