feat(chat/web): add explicit ChatButtonEntry interface and return typ…

[nitin-kumar-code]

Summary:
Add explicit TypeScript types to useChatButton in react/features/chat/hooks.web.ts to improve type safety and maintainability.

Changes:

• Introduced ChatButtonEntry interface.
• Typed the chat constant as ChatButtonEntry.
• Added explicit return type ChatButtonEntry | undefined to useChatButton.
• No functional changes.

Rationale:

• Makes the hook contract explicit.
• Improves IDE autocomplete and static analysis.
• Prevents accidental structural changes to the chat entry object.
• Keeps behaviour identical (type-only improvement).

Impact:

• Web-only.
• No runtime changes.
• No UI changes.
• No breaking changes.

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[nitin-kumar-code]

Hi, thank you for the reminder.
I have signed the Individual CLA. Please let me know if there are any further steps required from my side.
Looking forward to the review.