Make Chat Page Fully Mobile Friendly

[avishek0769]

Summary

Improve the chat experience on small screens so the page is usable end-to-end on phones and tablets.

Problem

The current chat layout is desktop-first with multiple side panels and dense controls. On mobile, key actions (message input, source view, doc links, model selection) can become cramped or hidden.

Expected Solution

Redesign the chat page responsive behavior for breakpoints below tablet sizes. Keep all major features available while simplifying interaction patterns for touch devices.

Scope

• src/pages/ChatPage.tsx
• src/components/Sidebar.tsx
• Shared styles/utilities used by chat layout

Acceptance Criteria

• Chat input, send action, and message list are fully usable on mobile widths.
• Left/right contextual panels are replaced with mobile-appropriate drawers/sheets/toggles.
• No horizontal overflow or clipped content on common device widths (320px, 375px, 390px, 414px).
• Sources panel and indexed-links modal remain accessible and readable on mobile.

Notes

Prioritize usability over keeping the exact desktop visual structure.