refactor(hivechat): centralize chat error/placeholder messages as constants

[aliraza556]

This PR centralizes repeated user-facing strings used in Hive Chat into a single constants object to improve consistency and maintainability.

Changes:

• Added ERROR_MESSAGES to src/people/utils/Constants.tsx
• Replaced hardcoded strings in src/people/hiveChat/index.tsx:
  • "Cannot Send Message" toast title
  • GitHub PAT expired message
  • Workspace setup incomplete message
  • "Type your message..." textarea placeholder

Why:

• Single source of truth for messages
• Easier future edits and localization
• Reduces risk of inconsistencies/typos

Notes:

• No behavioral changes; user-visible text remains the same
• Lint passes locally with no new issues

Testing:

• Open Hive Chat
• With incomplete setup or expired PAT, verify toast messages display correctly
• Verify the input placeholder shows the constant when applicable

[aliraza556]

@kevkevinpal, @tomsmith8 Please review this PR.