feat: add search term highlighting in search messages

[srijnabhargav]

Adds search-term highlighting in Search Messages, similar to Rocket.Chat.

Acceptance Criteria fulfillment

• Highlights matched words with a yellow background
• Works for plain text, inline code, links, bold/italic content
• No UI changes or new controls
• Does not mutate original message objects

Fixes #1043

Screenshots

PR Test Details

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-1044 after approval.