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/base.html

@@ -189,9 +189,9 @@
     <div class="container">
         {% if current_user and current_user.is_authenticated %}
         <div class="nav">
-            <a href="{{ url_for('index') }}">Home</a>
+            <a href="{{ url_for('landing.index') }}">Home</a>
             {% if current_user.role == 'product_owner' %}
-                <span style="color: #999;">(Dashboard - Coming in Phase 5)</span>
+                <a href="{{ url_for('dashboard.list') }}">Dashboard</a>
             {% elif current_user.role == 'administrator' %}
                 <span style="color: #999;">(Admin Panel - Coming in Phase 6)</span>
             {% endif %}