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/queue-sse.spec.ts

@@ -0,0 +1,141 @@
+/**
+ * Integration tests for Queue SSE Stream endpoint
+ * 
+ * Tests the Server-Sent Events stream for real-time queue updates.
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { queueManager } from '$lib/server/queue/QueueManager';
+import { GET as streamGET } from '../routes/api/queue/stream/+server.js';
+
+describe('Queue SSE Stream Endpoint', () => {
+  beforeEach(() => {
+    // Clear queue between tests
+    queueManager.getAll().forEach(item => queueManager.remove(item.id));
+  });
+  
+  afterEach(() => {
+    // Clean up after tests
+    queueManager.getAll().forEach(item => queueManager.remove(item.id));
+  });
+  
+  describe('GET /api/queue/stream', () => {
+    it('should return SSE response with correct headers', async () => {
+      const url = new URL('http://localhost/api/queue/stream');
+      const request = new Request(url);
+      
+      const response = await streamGET({ 
+        url, 
+        request: {
+          ...request,
+          signal: new AbortController().signal
+        }
+      } as any);
+      
+      expect(response.status).toBe(200);
+      expect(response.headers.get('Content-Type')).toBe('text/event-stream');
+      expect(response.headers.get('Cache-Control')).toBe('no-cache');
+      expect(response.headers.get('Connection')).toBe('keep-alive');
+    });
+    
+    it('should reject invalid status filter', async () => {
+      const url = new URL('http://localhost/api/queue/stream?status=invalid');
+      const request = new Request(url);
+      
+      const response = await streamGET({ 
+        url, 
+        request: {
+          ...request,
+          signal: new AbortController().signal
+        }
+      } as any);
+      
+      expect(response.status).toBe(400);
+      const text = await response.text();
+      expect(text).toContain('Invalid status filter');
+    });
+    
+    it('should reject invalid item ID format', async () => {
+      const url = new URL('http://localhost/api/queue/stream?id=invalid-id');
+      const request = new Request(url);
+      
+      const response = await streamGET({ 
+        url, 
+        request: {
+          ...request,
+          signal: new AbortController().signal
+        }
+      } as any);
+      
+      expect(response.status).toBe(400);
+      const text = await response.text();
+      expect(text).toBe('Invalid queue item ID format');
+    });
+    
+    it('should accept valid status filter', async () => {
+      const url = new URL('http://localhost/api/queue/stream?status=pending');
+      const request = new Request(url);
+      
+      const response = await streamGET({ 
+        url, 
+        request: {
+          ...request,
+          signal: new AbortController().signal
+        }
+      } as any);
+      
+      expect(response.status).toBe(200);
+      expect(response.headers.get('Content-Type')).toBe('text/event-stream');
+    });
+    
+    it('should accept valid item ID filter', async () => {
+      // Add a test item first
+      const item = queueManager.enqueue('https://instagram.com/p/TEST123');
+      
+      const url = new URL(`http://localhost/api/queue/stream?id=${item.id}`);
+      const request = new Request(url);
+      
+      const response = await streamGET({ 
+        url, 
+        request: {
+          ...request,
+          signal: new AbortController().signal
+        }
+      } as any);
+      
+      expect(response.status).toBe(200);
+      expect(response.headers.get('Content-Type')).toBe('text/event-stream');
+    });
+    
+    it('should handle stream initialization without errors', async () => {
+      // Add some test items
+      queueManager.enqueue('https://instagram.com/p/TEST1');
+      queueManager.enqueue('https://instagram.com/p/TEST2');
+      
+      const url = new URL('http://localhost/api/queue/stream');
+      const abortController = new AbortController();
+      const request = new Request(url, {
+        signal: abortController.signal
+      });
+      
+      const response = await streamGET({ 
+        url, 
+        request 
+      } as any);
+      
+      expect(response.status).toBe(200);
+      expect(response.body).toBeInstanceOf(ReadableStream);
+      
+      // Abort the request to clean up
+      abortController.abort();
+    });
+  });
+  
+  // Note: Full SSE stream testing would require more complex setup with
+  // ReadableStream readers and async iteration, which is beyond the scope
+  // of these basic endpoint validation tests. The above tests verify that:
+  // 1. The endpoint responds correctly
+  // 2. Headers are set properly for SSE
+  // 3. Parameter validation works
+  // 4. Stream initialization succeeds
+});