refactor: redesign UI to match EduAI Refined Workspace

- Replace Bauhaus aesthetic with Material Design 3-inspired layout
- Add working grade/subject/chapter dropdown selectors with placeholder options
- Fix ChatInput layout: remove position:absolute, use flex column flow
- Update sidebar with Material Icons, upgrade card, and clean hover states
- Add TopNav dropdowns with click-outside-to-close behavior
- Modernize message bubbles, action buttons, and welcome state
- Replace SVG icons with Material Symbols throughout
- Apply consistent BEM naming, CSS custom properties, no inline styles

frontend/src/main.jsx

@@ -1,5 +1,5 @@
 /* ========================================
-   Main Entry Point — padhLe
+   Main Entry Point — padhle
    ======================================== */
 
 import React from "react";