Make navbar fixed at top and add padding to lesson Back/Next buttons

[Saritahimthani]

No description provided.

[F-Hejazi]

Hey Sarita,
Thanks for the contribution! A few things to flag:

• The sticky → fixed change on the navbar isn't needed. sticky already keeps the navbar visible while scrolling and stays in the document flow, so the pt-24 added to App.tsx is only there to compensate for fixed removing the element from flow. The original behavior is the same, so both changes can be reverted.

• left-0 right-0 w-full on the fixed header are redundant. left-0 right-0 already stretches it to full width. Again, sticky elements naturally take the full width of their parent, so none of these are needed.

• For the hardcoded px values (gap: 12px, padding: 0 22px, etc.), we're actively converting these files from SCSS to inline Tailwind and removing the SCSS modules altogether, so it's best to avoid adding new hardcoded values in the meantime. Once we get to those files, everything will be moved to Tailwind utilities which handle responsiveness more cleanly.

[Saritahimthani]

Thank you for the clarification and review of the code.