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/routes/auth.py

@@ -28,11 +28,13 @@ def login():
             login_user(user)
             flash(f'Welcome back, {user.username}!', 'success')
 
-            # Redirect based on role
-            if user.role == 'administrator':
-                return redirect(url_for('admin.dashboard'))
-            elif user.role == 'product_owner':
-                return redirect(url_for('dashboard.list'))
+            # TODO: Redirect based on role when dashboards are implemented
+            # For now, redirect to index page
+            # if user.role == 'administrator':
+            #     return redirect(url_for('admin.dashboard'))
+            # elif user.role == 'product_owner':
+            #     return redirect(url_for('dashboard.list'))
+            return redirect(url_for('index'))
         else:
             flash('Invalid username or password', 'error')
 
@@ -45,4 +47,4 @@ def logout():
     """User logout"""
     logout_user()
     flash('You have been logged out', 'info')
-    return redirect(url_for('submission.form'))
+    return redirect(url_for('index'))