feat(web): add theme definition system with 5 built-in themes (#493)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/web/src/themes/registry.ts

@@ -0,0 +1,50 @@
+import { darkTheme } from "./dark";
+import { draculaTheme } from "./dracula";
+import { lightTheme } from "./light";
+import { nordTheme } from "./nord";
+import { solarizedDarkTheme } from "./solarized-dark";
+import type { ThemeDefinition } from "./types";
+
+/** All built-in themes, ordered for display */
+const builtInThemes: ThemeDefinition[] = [
+  darkTheme,
+  lightTheme,
+  nordTheme,
+  draculaTheme,
+  solarizedDarkTheme,
+];
+
+const themeMap = new Map<string, ThemeDefinition>(builtInThemes.map((t) => [t.id, t]));
+
+/** Default theme when no preference is set */
+export const DEFAULT_THEME_ID = "dark";
+
+/** Get all registered themes */
+export function getAllThemes(): ThemeDefinition[] {
+  return [...builtInThemes];
+}
+
+/** Get a theme by ID, or undefined if not found */
+export function getTheme(id: string): ThemeDefinition | undefined {
+  return themeMap.get(id);
+}
+
+/** Get a theme by ID, falling back to the default dark theme */
+export function getThemeOrDefault(id: string): ThemeDefinition {
+  return themeMap.get(id) ?? darkTheme;
+}
+
+/** Get only dark themes */
+export function getDarkThemes(): ThemeDefinition[] {
+  return builtInThemes.filter((t) => t.isDark);
+}
+
+/** Get only light themes */
+export function getLightThemes(): ThemeDefinition[] {
+  return builtInThemes.filter((t) => !t.isDark);
+}
+
+/** Check if a theme ID is valid */
+export function isValidThemeId(id: string): boolean {
+  return themeMap.has(id);
+}