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/panel.rs

@@ -0,0 +1,88 @@
+use gpui::prelude::*;
+
+use crate::theme::ActiveTheme;
+
+#[derive(gpui::IntoElement)]
+pub struct Panel {
+    content: Vec<gpui::AnyElement>,
+    title: Option<String>,
+    border: f32,
+    padding: f32,
+}
+
+impl Panel {
+    pub fn new() -> Self {
+        Self {
+            content: Vec::new(),
+            title: None,
+            border: 1.0,
+            padding: 8.0,
+        }
+    }
+
+    pub fn title(mut self, title: impl Into<String>) -> Self {
+        self.title = Some(title.into());
+        self
+    }
+
+    pub fn border(mut self, border: f32) -> Self {
+        self.border = border;
+        self
+    }
+
+    pub fn padding(mut self, padding: f32) -> Self {
+        self.padding = padding;
+        self
+    }
+
+    pub fn child(mut self, child: impl gpui::IntoElement) -> Self {
+        self.content.push(child.into_any_element());
+        self
+    }
+
+    pub fn children<E>(mut self, children: impl IntoIterator<Item = E>) -> Self
+    where
+        E: gpui::IntoElement,
+    {
+        self.content
+            .extend(children.into_iter().map(|c| c.into_any_element()));
+        self
+    }
+}
+
+impl gpui::RenderOnce for Panel {
+    fn render(mut self, _window: &mut gpui::Window, cx: &mut gpui::App) -> impl IntoElement {
+        let theme = cx.theme();
+
+        let header = self.title.as_ref().map(|title| {
+            gpui::div()
+                .px_2()
+                .py_1()
+                .text_sm()
+                .text_color(theme.muted)
+                .border_b(gpui::px(self.border))
+                .border_color(theme.border)
+                .child(title.clone())
+                .into_any_element()
+        });
+
+        let children: Vec<gpui::AnyElement> = self
+            .content
+            .drain(..)
+            .enumerate()
+            .map(|(ix, c)| gpui::div().id(ix).child(c).into_any_element())
+            .collect();
+
+        gpui::div()
+            .size_full()
+            .bg(theme.panel)
+            .border(gpui::px(self.border))
+            .border_color(theme.border)
+            .rounded_md()
+            .p(gpui::px(self.padding))
+            .flex()
+            .flex_col()
+            .children(header)
+            .children(children)
+    }
+}