Fix url_for references and improve navigation

Issues fixed:
- Fixed base.html to use url_for('landing.index') instead of url_for('index')
- Fixed error_403.html and error_404.html to use correct landing page endpoint
- Added Dashboard link in base.html nav for product owners
- Removed obsolete templates/index.html (replaced by landing page)
- Added dashboard link in landing/index.html for authenticated product owners
- Added test to verify landing page renders correctly with proper url_for references

All 8 landing page tests passing.

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

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

app/templates/landing/index.html

@@ -8,6 +8,16 @@
 {% if products %}
     <p>Select a product to share your feedback, report issues, or suggest improvements.</p>
 
+    {% if current_user and current_user.is_authenticated and current_user.role == 'product_owner' %}
+        <div style="margin-top: 20px; padding: 15px; background-color: #e7f3ff; border-radius: 4px; border-left: 4px solid #2196F3;">
+            <p style="margin: 0;">
+                <strong>Product Owner:</strong>
+                <a href="{{ url_for('dashboard.list') }}" style="color: #1976D2; text-decoration: underline;">Go to Dashboard</a>
+                to view and analyze feedback.
+            </p>
+        </div>
+    {% endif %}
+
     <div style="margin-top: 30px;">
         {% for product in products %}
             <div style="background-color: #f8f9fa; padding: 20px; border-radius: 4px; margin-bottom: 15px; border-left: 4px solid #3498db;">