feat: add dev-only visual theme builder toolbar

Floating palette button (bottom-left) opens a panel to live-preview:
- Theme presets (switches data-preset + fonts + accent)
- Dark/light mode
- Photo frame style
- Film grain toggle
- Grid columns/gap sliders

Hero styles and grid layouts copy YAML to clipboard (need server re-render).
Only renders when NODE_ENV=development. Toggle: click or Cmd+Shift+T.

app/layout.tsx

@@ -11,6 +11,7 @@ import { SubpageNav } from '@/components/SubpageNav';
 import { ThemeToggle } from '@/components/ThemeToggle';
 import { ScrollToTop } from '@/components/ScrollToTop';
 import { Footer } from '@/components/Footer';
+import { DevToolbar } from '@/components/DevToolbar';
 import { getConfig, getGoogleFontsUrl } from '@/lib/config';
 
 const siteTitle = process.env.SITE_TITLE || 'Gallery';
@@ -83,6 +84,7 @@ export default function RootLayout({ children }: { children: React.ReactNode })
         <main className="main">{children}</main>
         <Footer />
         <ScrollToTop />
+        {process.env.NODE_ENV === 'development' && <DevToolbar />}
       </body>
     </html>
   );