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/e2e/projects-list.spec.ts

@@ -125,6 +125,7 @@ test.describe('Projects List Page', () => {
 
   test('should be accessible from dashboard', async ({ page }) => {
     await loginViaUI(page);
+    await page.goto('/en/dashboard');
     await page.waitForLoadState('networkidle');
 
     // Click "View all" link from dashboard