fix(frontend): move dashboard to /dashboard route

The dashboard page was created at (authenticated)/page.tsx which would
serve the same route as [locale]/page.tsx (the public landing page).
Next.js doesn't allow route groups to override parent pages.

Changes:
- Move dashboard page to (authenticated)/dashboard/page.tsx
- Update Header nav links to point to /dashboard
- Update AppBreadcrumbs home link to /dashboard
- Update E2E tests to navigate to /dashboard

Now authenticated users should navigate to /dashboard for their homepage,
while /en serves the public landing page for unauthenticated users.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/components/layout/Header.tsx

@@ -83,7 +83,7 @@ export function Header() {
       <div className="container mx-auto flex h-16 items-center px-4">
         {/* Logo */}
         <div className="flex items-center space-x-8">
-          <Link href="/" className="flex items-center space-x-2">
+          <Link href="/dashboard" className="flex items-center space-x-2">
             <Image
               src="/logo-icon.svg"
               alt="Syndarix Logo"
@@ -96,7 +96,7 @@ export function Header() {
 
           {/* Navigation Links */}
           <nav className="hidden md:flex items-center space-x-1">
-            <NavLink href="/" exact>
+            <NavLink href="/dashboard" exact>
               {t('home')}
             </NavLink>
             {user?.is_superuser && <NavLink href="/admin">{t('admin')}</NavLink>}