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

Create the theme architecture layer for MS18: TypeScript theme
definitions, theme registry, and 5 built-in themes (Dark, Light,
Nord, Dracula, Solarized Dark). Each theme provides complete CSS
variable overrides via a typed ThemeDefinition interface.

Refs: #487

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/themes/index.ts

@@ -0,0 +1,18 @@
+export type { ThemeColors, ThemeDefinition, ThemeShadows, ThemeColorKey } from "./types";
+export { themeToVariables } from "./types";
+
+export { darkTheme } from "./dark";
+export { lightTheme } from "./light";
+export { nordTheme } from "./nord";
+export { draculaTheme } from "./dracula";
+export { solarizedDarkTheme } from "./solarized-dark";
+
+export {
+  getAllThemes,
+  getTheme,
+  getThemeOrDefault,
+  getDarkThemes,
+  getLightThemes,
+  isValidThemeId,
+  DEFAULT_THEME_ID,
+} from "./registry";