Show "You:" prefix for last message in conversation list

[joelmuraguri]

Closes #1317

What changed:

• Updated Conversation.summary() to accept signedInProfileId and compare against message.sender.did
• When the signed-in user sent the last message, the summary now prefixes with You:
• Added you_sent_prefix string resource
• Threaded signedInProfileId from the Conversation composable down into the summary() call

[gemini-code-assist]

Code Review

This pull request adds a "You: " prefix to the conversation summary when the last message was sent by the signed-in user. The review feedback recommends using a format string instead of manual concatenation and trailing spaces in the string resource to improve localization support.