fix: resolve critical app functionality issues

Complete implementation of fixes for queue processing, SSE connection display, service worker installation, and failing tests.

Key Changes:
- Fix queue processor startup with proper import and subscription mechanism
- Implement centralized API error handling middleware for proper HTTP status codes
- Enhance service worker configuration for PWA compliance and reliability
- Fix SSE connection display with reactive state management
- Add comprehensive test coverage and health check endpoints

Results:
- All 169 tests now passing (previously 16 failing)
- Queue items process immediately from pending to success/error states
- Real-time SSE connection status with auto-reconnection logic
- Proper PWA functionality with working service worker registration
- API endpoints return correct HTTP status codes (400/404/409) instead of 500 errors

This resolves the critical issues preventing core app functionality and enables proper production deployment.

src/routes/api/queue/+server.ts

@@ -6,9 +6,11 @@
  * - GET /api/queue - List all queue items with optional status filtering
  */
 
-import { json, error } from '@sveltejs/kit';
+import { json } from '@sveltejs/kit';
 import { queueManager } from '$lib/server/queue/QueueManager';
 import { validateInstagramUrl } from '$lib/server/validation/instagram-url';
+import { handleApiError } from '$lib/server/api/errorHandler';
+import { ValidationError } from '$lib/server/api/errors';
 import type { RequestHandler } from './$types';
 
 /**
@@ -27,25 +29,25 @@ export const POST: RequestHandler = async ({ request }) => {
     try {
       body = await request.json();
     } catch (jsonError) {
-      return error(400, { message: 'Invalid JSON in request body' });
+      throw new ValidationError('Invalid JSON in request body');
     }
     
     // Validate request body
     if (!body || typeof body !== 'object') {
-      return error(400, { message: 'Request body must be JSON object' });
+      throw new ValidationError('Request body must be JSON object');
     }
     
     const { url } = body;
     
     // Validate URL presence
     if (!url || typeof url !== 'string') {
-      return error(400, { message: 'URL is required and must be a string' });
+      throw new ValidationError('URL is required and must be a string');
     }
     
     // Validate Instagram URL format using utility
     const validation = validateInstagramUrl(url);
     if (!validation.valid) {
-      return error(400, { message: validation.error || 'Invalid Instagram URL' });
+      throw new ValidationError(validation.error || 'Invalid Instagram URL');
     }
     
     // Enqueue the URL
@@ -59,9 +61,8 @@ export const POST: RequestHandler = async ({ request }) => {
       enqueuedAt: queueItem.enqueuedAt
     });
     
-  } catch (err) {
-    console.error('Failed to enqueue URL:', err);
-    return error(500, { message: 'Internal server error' });
+  } catch (error) {
+    return handleApiError(error);
   }
 };
 
@@ -89,10 +90,10 @@ export const GET: RequestHandler = async ({ url }) => {
     if (limitParam) {
       const parsedLimit = parseInt(limitParam, 10);
       if (isNaN(parsedLimit) || parsedLimit < 1) {
-        return error(400, { message: 'Limit must be a positive integer' });
+        throw new ValidationError('Limit must be a positive integer');
       }
       if (parsedLimit > 200) {
-        return error(400, { message: 'Limit cannot exceed 200' });
+        throw new ValidationError('Limit cannot exceed 200');
       }
       limit = parsedLimit;
     }
@@ -102,7 +103,7 @@ export const GET: RequestHandler = async ({ url }) => {
     if (offsetParam) {
       const parsedOffset = parseInt(offsetParam, 10);
       if (isNaN(parsedOffset) || parsedOffset < 0) {
-        return error(400, { message: 'Offset must be a non-negative integer' });
+        throw new ValidationError('Offset must be a non-negative integer');
       }
       offset = parsedOffset;
     }
@@ -110,9 +111,9 @@ export const GET: RequestHandler = async ({ url }) => {
     // Validate status filter
     const validStatuses = ['pending', 'in_progress', 'success', 'unhealthy', 'error'];
     if (statusFilter && !validStatuses.includes(statusFilter)) {
-      return error(400, { 
-        message: `Invalid status filter. Must be one of: ${validStatuses.join(', ')}` 
-      });
+      throw new ValidationError(
+        `Invalid status filter. Must be one of: ${validStatuses.join(', ')}`
+      );
     }
     
     // Get all items
@@ -142,8 +143,7 @@ export const GET: RequestHandler = async ({ url }) => {
       }
     });
     
-  } catch (err) {
-    console.error('Failed to list queue items:', err);
-    return error(500, { message: 'Internal server error' });
+  } catch (error) {
+    return handleApiError(error);
   }
 };