Implement Vertical Sidebar Menu with Payments Section and Responsive Design

This pull request transforms the horizontal main menu into a vertical sidebar menu. The changes include the following:

Adds a new "Payments" section to the sidebar menu with submenu items "yno AI" and "Yanok Access" linking to the Stripe payment page.
Adjusts indentation for menu items and makes entire menu lines clickable.
Updates 'Solutions' to 'Use Cases' and removes the "By Industry" and "By Role" subheadings.
Updates the sidebar collapse button to hide the menu content completely instead of squishing it.
Ensures that the logo remains visible and the sidebar collapse button is repositioned for mobile.
Introduces a fade effect for scrollbars and makes scrollbars only appear when content doesn't fit the screen.
Adjusts the main content side indent to 8% to accommodate the new sidebar menu.
Ensures all updates are responsive and function properly across devices and screen sizes.

These changes ensure a better user experience while maintaining responsiveness and accessibility.

Tips:

Make sure to test changes before merging.
Submit a "Request Changes" review and I'll address it.
Close this PR with a comment if it's obviously incorrect. This will improve my future PRs.
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yanok-stage / www.yanok.ai