WIP: Add theming system with /theme command

- Consolidated theme system into single src/theme/ directory
- Created Theme class with fg(), bg(), bold(), italic(), underline()
- Added dark and light built-in themes with 36 color tokens
- Support for custom themes in ~/.pi/agent/themes/*.json
- JSON schema for theme validation
- Theme selector UI with /theme command
- Save theme preference to settings
- Uses chalk for text formatting to preserve colors

TODO:
- Replace hardcoded colors throughout TUI components
- Apply markdown theming to Markdown components
- Add theme support to all TUI elements

packages/coding-agent/src/tui/theme-selector.ts

@@ -0,0 +1,51 @@
+import { Container, type SelectItem, SelectList } from "@mariozechner/pi-tui";
+import { getAvailableThemes, theme } from "../theme/theme.js";
+import { DynamicBorder } from "./dynamic-border.js";
+
+/**
+ * Component that renders a theme selector
+ */
+export class ThemeSelectorComponent extends Container {
+	private selectList: SelectList;
+
+	constructor(currentTheme: string, onSelect: (themeName: string) => void, onCancel: () => void) {
+		super();
+
+		// Get available themes and create select items
+		const themes = getAvailableThemes();
+		const themeItems: SelectItem[] = themes.map((name) => ({
+			value: name,
+			label: name,
+			description: name === currentTheme ? "(current)" : undefined,
+		}));
+
+		// Add top border
+		this.addChild(new DynamicBorder((text) => theme.fg("border", text)));
+
+		// Create selector
+		this.selectList = new SelectList(themeItems, 10);
+
+		// Preselect current theme
+		const currentIndex = themes.indexOf(currentTheme);
+		if (currentIndex !== -1) {
+			this.selectList.setSelectedIndex(currentIndex);
+		}
+
+		this.selectList.onSelect = (item) => {
+			onSelect(item.value);
+		};
+
+		this.selectList.onCancel = () => {
+			onCancel();
+		};
+
+		this.addChild(this.selectList);
+
+		// Add bottom border
+		this.addChild(new DynamicBorder((text) => theme.fg("border", text)));
+	}
+
+	getSelectList(): SelectList {
+		return this.selectList;
+	}
+}