bug : Standardize Design Tokens and Resolve Navigation Accessibility Gaps

### **Description :**
Currently, the frontend contains several hardcoded hex values for colors (e.g., #212529 and #87a1bc) and accessibility oversights in the navigation components that hinder screen reader and keyboard-only users.

### **Changes Proposed:**

Design Tokens: Centralized brand-dark (#212529) and blue-light (#87a1bc) colors in tailwind.config.mjs to ensure visual consistency across Card, Modal, and UserMenu.
A11y (Navigation): Implemented a keyboard focus trap in NavDropDown to prevent focus from escaping the sub-menus unexpectedly.
A11y (Semantic HTML): Updated Footer.tsx to replace "h3" tags with "span" in buttons, resolving redundant heading levels in the document outline.
A11y (Attributes): Added personalized alt text for user avatars in UserMenu and supported aria-hidden attributes for decorative icons.

### **Impact:**
Ensures WCAG compliance for navigation and simplifies future theme updates through centralized tokens.