fix: polish form validation and landing page styling

[nasserx]

Summary

• Improved validation error styling so invalid form fields use a red border without applying an internal red/brown background tint.
• Centered the main landing page navigation links within the top navbar.
• Improved active section highlighting while scrolling to make the nav state more stable.
• Removed unnecessary landing page visual clutter where applicable.
• Refined the landing preview/dashboard card styling.
• Removed the preview card outer border in dark mode only while preserving the light mode appearance.

Notes

• This change is limited to UI polish and frontend styling behavior.
• No Flask routes, backend logic, database models, or dependencies were changed.
• No release is required for this UI-only update.

Checks

• Verified invalid form fields keep their normal internal background in both light and dark mode.
• Verified landing navigation behavior while scrolling and clicking nav links.
• Verified hover and active nav states in light and dark mode.
• Verified the preview/dashboard card remains visually consistent with other landing cards.
• Verified light mode card styling was not changed unintentionally.