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/app.rs

@@ -0,0 +1,82 @@
+use gpui::prelude::*;
+use std::sync::Arc;
+
+use crate::components::{
+    self,
+    input::{Input, Suggestion},
+};
+
+pub(crate) struct App {
+    input: gpui::Entity<Input>,
+    current_text: gpui::SharedString,
+}
+
+impl gpui::Render for App {
+    fn render(
+        &mut self,
+        _window: &mut gpui::Window,
+        _cx: &mut gpui::Context<Self>,
+    ) -> impl gpui::IntoElement {
+        components::panel::Panel::new()
+            .child(self.input.clone())
+            .child(components::label::Label::new(format!(
+                "Text: {}",
+                &self.current_text
+            )))
+    }
+}
+
+impl App {
+    pub fn run() -> () {
+        let app = gpui::Application::new();
+
+        app.run(|app: &mut gpui::App| {
+            app.set_global(crate::theme::Theme::dark());
+            app.open_window(
+                gpui::WindowOptions::default(),
+                |_window: &mut gpui::Window, app: &mut gpui::App| {
+                    app.new(|cx: &mut gpui::Context<'_, App>| {
+                        let input = cx.new(|cx| {
+                            Input::new("input", cx, "Type here...").with_suggest(Arc::new(|text| {
+                                let suggestions = vec![
+                                    ("project:", "Filter by project"),
+                                    ("+work", "Tag: work"),
+                                    ("+personal", "Tag: personal"),
+                                    ("due:today", "Tasks for today"),
+                                    ("due:tomorrow", "Tasks for tomorrow"),
+                                    ("priority:H", "Priority high"),
+                                    ("priority:M", "Priority medium"),
+                                    ("priority:L", "Priority low"),
+                                ];
+
+                                suggestions
+                                    .into_iter()
+                                    .filter(|(insert, _)| {
+                                        text.is_empty()
+                                            || insert.to_lowercase().contains(&text.to_lowercase())
+                                    })
+                                    .map(|(insert, label)| {
+                                        Suggestion::new(format!("{} - {}", insert, label), insert)
+                                    })
+                                    .collect()
+                            }))
+                        });
+
+                        cx.observe(&input, |this, input, cx| {
+                            let value = input.read(cx).value().to_string();
+                            this.current_text = value.into();
+                            cx.notify();
+                        })
+                        .detach();
+
+                        App {
+                            input,
+                            current_text: "".into(),
+                        }
+                    })
+                },
+            )
+            .unwrap();
+        });
+    }
+}