feat: add Umami analytics with tagged events

Add Umami web analytics tracking script to the layout head, and
instrument all interactive elements with event tracking:

Declarative (data-umami-event attributes):
- Category selector: select_category
- Style selector: select_style
- Intensity slider: adjust_intensity
- Prompt toggle: toggle_prompt (with open/close action)

Programmatic (umami.track with metadata):
- convert_click: { style, intensity }
- convert_success: { style, intensity, model }
- convert_error: { style, intensity, error }
- copy_result: { style }

src/routes/+layout.svelte

@@ -6,6 +6,7 @@
 
 <svelte:head>
 	<link rel="icon" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>✨</text></svg>" />
+	<script defer src="https://wa.santhoshj.com/script.js" data-website-id="1004961a-2b1f-4c45-95a6-78059acac472"></script>
 </svelte:head>
 
 {@render children()}