fix(01-code-splitting): convert billing, blog, content-planning, error-boundary, pricing, alerts MUI icons

- Converted barrel imports to individual imports across 8 files
- Affected files: billing (2), BlogWriter (1), ContentPlanningDashboard (2), ErrorBoundary (1), Pricing (1), AlertsBadge (1)

frontend/src/components/BlogWriter/BlogWriterPhasesSection.tsx

@@ -1,6 +1,7 @@
 import React, { useState } from 'react';
 import { Container, Grid, Card, CardContent, Typography, Box, Stack, Chip } from '@mui/material';
-import { CheckCircle, AutoAwesome } from '@mui/icons-material';
+import CheckCircle from '@mui/icons-material/CheckCircle';
+import AutoAwesome from '@mui/icons-material/AutoAwesome';
 
 interface PhaseFeature {
   title: string;