SegmentedControl: only fire onValueChange on actual changes

Prevents onValueChange from firing during initialization, which
caused pushState errors before router was ready.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/lib/components/ui/segmented-control/SegmentedControl.svelte

@@ -37,9 +37,16 @@
 	// Provide variant to child segments via context
 	setContext('segmented-control-variant', variant)
 
+	// Track previous value to only fire callback on actual changes (not initialization)
+	let previousValue = $state<string | undefined>(undefined)
+
 	$effect(() => {
 		if (onValueChange && value !== undefined) {
-			onValueChange(value)
+			// Only call onValueChange if value actually changed (not on initialization)
+			if (previousValue !== undefined && value !== previousValue) {
+				onValueChange(value)
+			}
+			previousValue = value
 		}
 	})