feat(web): add navigation menu bar for logged-in users

- Add horizontal nav menu with 7 items: Profile, Characters, Sessions,
  Mechanics, Leaderboard, Settings, Help
- Implement responsive hamburger menu for mobile (≤768px)
- Create pages blueprint with stub routes for new pages
- Add "Coming Soon" styled stub templates with icons
- Include active state highlighting for current page

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

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

public_web/app/__init__.py

@@ -57,10 +57,12 @@ def create_app():
     from .views.auth_views import auth_bp
     from .views.character_views import character_bp
     from .views.game_views import game_bp
+    from .views.pages import pages_bp
 
     app.register_blueprint(auth_bp)
     app.register_blueprint(character_bp)
     app.register_blueprint(game_bp)
+    app.register_blueprint(pages_bp)
 
     # Register Jinja filters
     def format_timestamp(iso_string: str) -> str:
@@ -107,6 +109,6 @@ def create_app():
         logger.error("internal_server_error", error=str(error))
         return render_template('errors/500.html'), 500
 
-    logger.info("flask_app_created", blueprints=["auth", "character", "game"])
+    logger.info("flask_app_created", blueprints=["auth", "character", "game", "pages"])
 
     return app