Use official PriceGhost icon in header and login page

- Replace ghost emoji with SVG icon in navbar
- Replace ghost emoji with SVG icon on login/register page
- Consistent branding throughout the app

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/AuthForm.tsx

@@ -92,7 +92,8 @@ export default function AuthForm({ mode, onSubmit }: AuthFormProps) {
         }
 
         .auth-form-logo {
-          font-size: 2.5rem;
+          width: 64px;
+          height: 64px;
           margin-bottom: 0.5rem;
         }
 
@@ -146,7 +147,7 @@ export default function AuthForm({ mode, onSubmit }: AuthFormProps) {
 
       <div className="auth-form-card">
         <div className="auth-form-header">
-          <div className="auth-form-logo">👻</div>
+          <img src="/icon.svg" alt="PriceGhost" className="auth-form-logo" />
           <h1 className="auth-form-title">PriceGhost</h1>
           <p className="auth-form-subtitle">
             {mode === 'login'

frontend/src/components/Layout.tsx

@@ -105,7 +105,8 @@ export default function Layout({ children }: LayoutProps) {
         }
 
         .navbar-brand-icon {
-          font-size: 1.5rem;
+          width: 28px;
+          height: 28px;
         }
 
         .navbar-user {
@@ -264,7 +265,7 @@ export default function Layout({ children }: LayoutProps) {
       <nav className="navbar">
         <div className="navbar-content">
           <Link to="/" className="navbar-brand">
-            <span className="navbar-brand-icon">👻</span>
+            <img src="/icon.svg" alt="" className="navbar-brand-icon" />
             <span>PriceGhost</span>
           </Link>