Refresh frontend UI: typography, colors, accessibility, and responsiv…

[samkeen]

…e layout

Design improvements:

• Add IBM Plex Sans/Mono fonts for distinctive, readable typography
• Replace blue accent with warm orange color palette (#ea580c)
• Add CSS design tokens for consistent spacing, shadows, and transitions
• Improve message bubbles with gradients, shadows, and slide-in animation

Accessibility:

• Add skip link for keyboard navigation (hidden until focused)
• Add ARIA landmarks, roles, and live regions to HTML
• Convert help icons to buttons with 44px touch targets (WCAG)
• Add :focus-visible states to all interactive elements
• Add proper ARIA attributes to modals and tabs

Responsive layout:

• Add mobile sidebar as bottom drawer (< 768px)
• Add hamburger menu toggle with JS handler
• Make settings tabs horizontally scrollable on mobile
• Add media queries for tablet (768-1024px) and large screens (> 1200px)

Bug fixes:

• Fix sidebar overflow (now scrollable to access RAG section)
• Align info box colors with orange accent theme