fix(01-code-splitting): convert ImageStudio, Landing, LinkedIn, MainDashboard, OnboardingWizard MUI icons

- Converted barrel imports to individual imports across 14 files
- Most complex: VoiceAvatarPlaceholder (22 icons), FeatureShowcase (8), TestPersonaModal (9)

frontend/src/components/OnboardingWizard/ApiKeyStep.tsx

@@ -8,7 +8,7 @@ import {
   Grid,
   Snackbar,
 } from '@mui/material';
-import { Lock } from '@mui/icons-material';
+import Lock from '@mui/icons-material/Lock';
 import OnboardingButton from './common/OnboardingButton';
 import { 
   HelpSection,