Navigation menu

• Add inline menu buttons in the bot
• Implement sections: "О нас", "Тарифы", "Кейсы", "Свой вопрос"
• Handle menu navigation and back buttons

📋 Acceptance Criteria

✅ What to consider:

• Users should be able to easily navigate the bot using menu buttons.
• All sections ("О нас", "Тарифы", "Кейсы", "Свой вопрос") must be implemented and reachable from the main menu.
• Navigation should feel seamless, including return/back functionality.

✅ Acceptance Criteria (AC):

• GIVEN the user opens the bot
  WHEN the start message is shown
  THEN the main inline menu appears with buttons for "О нас", "Тарифы", "Кейсы", "Свой вопрос"

• GIVEN the user clicks on any menu button
  WHEN the bot receives the click
  THEN the corresponding section is opened
  AND the user sees the relevant content immediately

• GIVEN the user is inside any section
  WHEN they click the "Back" button
  THEN they are returned to the main menu without delay

✅ Please check the Definition of Done before moving this issue to Close.