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/tests/scheduler.integration.spec.ts

@@ -1,4 +1,4 @@
-import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
 import path from 'path';
 import fs from 'fs';