fix(ssr): resolve EventSource SSR violations and implement best practices

- Fix EventSource is not defined error in queue dashboard
- Add browser guards for all EventSource usage
- Replace static constants (EventSource.OPEN/CLOSED) with numeric values
- Fix setInterval SSR violation in LLM health indicator
- Replace $effect anti-pattern with onMount in share page
- Add comprehensive SvelteKit SSR best practices documentation
- Add SSR audit and testing verification

All changes follow SvelteKit best practices and are verified against
official documentation. Production build succeeds with no SSR errors.

Closes: FixEventSourceSSR
See: docs/outcomes/FixEventSourceSSR.md

src/routes/share/components/LlmHealthIndicator.svelte

@@ -1,4 +1,6 @@
 <script lang="ts">
+	import { onMount } from 'svelte';
+
 	interface HealthState {
 		status: 'checking' | 'healthy' | 'unhealthy' | 'error';
 		message: string;
@@ -33,7 +35,9 @@
 		}
 	}
 
-	$effect(() => {
+	// Use onMount instead of $effect for timer-based side effects
+	// onMount only runs in browser, no SSR guard needed
+	onMount(() => {
 		checkHealth(); // Initial check
 		const interval = setInterval(checkHealth, pollInterval);
 		return () => clearInterval(interval);