feat: integrate poyraz-ui and reactive-switcher for theme management

- Added poyraz-ui preset CSS import to globals.css for styling.
- Created theme-switcher.md documentation for reactive-switcher usage.
- Added usage-guide.md for comprehensive Poyraz UI instructions.
- Updated package.json to include reactive-switcher as a dependency.
- Modified pnpm-lock.yaml to reflect the addition of reactive-switcher.
- Established themes.ts to export Poyraz UI themes for use in the application.

src/lib/themes.ts

@@ -0,0 +1,15 @@
+import { poyrazLightTheme, poyrazDarkTheme, poyrazThemes } from "poyraz-ui/themes";
+
+// You can customise themes by spreading and overriding variables:
+//
+// const customLight = {
+//   ...poyrazLightTheme,
+//   name: "custom-light",
+//   variables: {
+//     ...poyrazLightTheme.variables,
+//     "--poyraz-primary": "#2563eb",            // blue-600
+//     "--poyraz-primary-foreground": "#ffffff",
+//   },
+// };
+
+export { poyrazLightTheme, poyrazDarkTheme, poyrazThemes };