feat: WCAG 2.1 accessibility compliance

[deepak0x]

Summary

• Skip-to-content link (visible on Tab, jumps to chat area)
• Logical tab order through header, message list, formatting toolbar, input, send button
• Visible focus indicators on all interactive elements
• aria-labels on all icon-only buttons (send, emoji, mic, video, pin, star, thread, etc.)
• role="toolbar" and aria-label on formatting toolbar with aria-expanded on overflow menu
• role="banner" and aria-label on chat header
• aria-live="polite" on audio recording timer for screen reader announcements
• Login form: label-input associations via htmlFor, aria-describedby on error messages
• role="dialog" on emoji picker
• Keyboard navigation hook (useKeyboardNav) for MessageToolbox

Demo

https://drive.google.com/drive/folders/1zfqEIHEX675Q-EnYfPCo8oStMw9g74HO