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/panel.rs

@@ -0,0 +1,82 @@
+use ratatui::{
+    buffer::Buffer,
+    layout::Rect,
+    style::{Color, Style},
+    widgets::{Block, Borders, Widget},
+};
+
+use crate::ui::app::AppMode;
+
+/// Trait for panel-specific content. Implement this to create a new side panel.
+pub trait PanelContent {
+    /// Whether the panel should appear active given the current mode.
+    fn is_active(&self, mode: &AppMode) -> bool;
+    /// Color used for the active border AND the selection highlight background.
+    fn active_color(&self) -> Color;
+    /// Block title string (include surrounding spaces for padding).
+    fn title(&self) -> &str;
+    /// Number of renderable rows.
+    fn item_count(&self) -> usize;
+    /// Message shown when `item_count()` returns 0.
+    fn empty_message(&self) -> &str;
+    /// Render a single item at the given row index.
+    /// `inner` is the full inner area of the panel; the item occupies row `index`.
+    fn render_item(&self, index: usize, is_selected: bool, inner: Rect, buf: &mut Buffer);
+    /// Optional footer rendered below the items (e.g. inline hints).
+    fn render_footer(&self, _inner: Rect, _buf: &mut Buffer) {}
+}
+
+/// Generic side-panel widget that delegates content rendering to a `PanelContent` impl.
+pub struct Panel<'a, C: PanelContent> {
+    content: C,
+    mode: &'a AppMode,
+    cursor: usize,
+}
+
+impl<'a, C: PanelContent> Panel<'a, C> {
+    pub fn new(content: C, mode: &'a AppMode, cursor: usize) -> Self {
+        Self {
+            content,
+            mode,
+            cursor,
+        }
+    }
+}
+
+impl<C: PanelContent> Widget for Panel<'_, C> {
+    fn render(self, area: Rect, buf: &mut Buffer) {
+        let is_active = self.content.is_active(self.mode);
+        let border_style = if is_active {
+            Style::default().fg(self.content.active_color())
+        } else {
+            Style::default().fg(Color::DarkGray)
+        };
+
+        let block = Block::default()
+            .borders(Borders::ALL)
+            .border_style(border_style)
+            .title(self.content.title());
+        let inner = block.inner(area);
+        block.render(area, buf);
+
+        if self.content.item_count() == 0 {
+            buf.set_string(
+                inner.x,
+                inner.y,
+                self.content.empty_message(),
+                Style::default().fg(Color::DarkGray),
+            );
+            return;
+        }
+
+        for i in 0..self.content.item_count() {
+            if i as u16 >= inner.height {
+                break;
+            }
+            let is_selected = i == self.cursor && is_active;
+            self.content.render_item(i, is_selected, inner, buf);
+        }
+
+        self.content.render_footer(inner, buf);
+    }
+}