feat: Add GPUI component library

Add core UI components for TaskWarrior GUI application:

Components:
- Input: Text input with autocomplete suggestions, keyboard navigation
- Button: Multiple variants (Primary, Secondary, Ghost, Danger, Success,
  Text)
- DropdownButton: Compound button with dropdown menu
- Icon: SVG icons with customizable sizes
- Label: Simple text display
- Panel: Container with optional title
- List: List container
- Modal: Modal dialog
- Divider: Visual separator

Infrastructure:
- Theme system with dark/light modes (Catppuccin colors)
- App module with window management

src/components/input/suggestion.rs

@@ -0,0 +1,24 @@
+use gpui::SharedString;
+
+#[derive(Clone, Debug)]
+pub struct Suggestion {
+    pub label: SharedString,
+    pub insert: SharedString,
+}
+
+impl Suggestion {
+    pub fn new(label: impl Into<SharedString>, insert: impl Into<SharedString>) -> Self {
+        Self {
+            label: label.into(),
+            insert: insert.into(),
+        }
+    }
+
+    pub fn simple(text: impl Into<SharedString>) -> Self {
+        let text = text.into();
+        Self {
+            label: text.clone(),
+            insert: text,
+        }
+    }
+}