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/mod.rs

@@ -6,5 +6,6 @@ pub mod formula_panel;
 pub mod grid;
 pub mod help;
 pub mod import_wizard_ui;
+pub mod panel;
 pub mod tile_bar;
 pub mod view_panel;