Fix BuildError for non-existent dashboard routes after login

After successful login, the app tried to redirect to admin.dashboard
or dashboard.list routes that don't exist yet (Phase 5 & 6).

Changes:
- Login now redirects to index page for all users
- Logout redirects to index page instead of submission.form
- Base template navigation shows "Coming in Phase X" messages
  instead of broken links to unimplemented routes
- Added TODO comments for future dashboard implementation

This allows login/logout to work properly in MVP (Phase 3) while
dashboard features are pending implementation.

Bug: werkzeug.routing.exceptions.BuildError: Could not build url
for endpoint 'admin.dashboard'

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

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

app/templates/base.html

@@ -189,13 +189,11 @@
     <div class="container">
         {% if current_user and current_user.is_authenticated %}
         <div class="nav">
+            <a href="{{ url_for('index') }}">Home</a>
             {% if current_user.role == 'product_owner' %}
-                <a href="{{ url_for('dashboard.list') }}">Dashboard</a>
-                <a href="{{ url_for('dashboard.products') }}">Products</a>
+                <span style="color: #999;">(Dashboard - Coming in Phase 5)</span>
             {% elif current_user.role == 'administrator' %}
-                <a href="{{ url_for('admin.dashboard') }}">Admin Dashboard</a>
-                <a href="{{ url_for('admin.users') }}">Users</a>
-                <a href="{{ url_for('admin.products') }}">Products</a>
+                <span style="color: #999;">(Admin Panel - Coming in Phase 6)</span>
             {% endif %}
             <a href="{{ url_for('auth.logout') }}" style="float: right;">Logout ({{ current_user.username }})</a>
         </div>