feat: add unread badge count for conversations

[naveensingh]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

• Added a colored badge to indicate the number of unread messages in conversations. This works similarly to other messaging apps except for how mark as read and mark as unread are handled.
• When a thread has unread items, the datetime text view is now also highlighted.
• Added call to markThreadMessagesRead() in ThreadActivity.onResume() so all messages are marked as read as soon as users enter the chat.

Tests performed

• Receiving and sending messages
• Mark/unmark as read

Before & after preview

Closes the following issue(s)

• Closes Poor visibility of unread messages on dark themes #177
• Closes Unread messages aren't marked as read on opening #264

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• All checks are passing.