feat: support @mentions in the send message API

[EmilioNicolas]

What

Adds optional mentions support to the bridge's POST /api/send endpoint, so text messages can tag users with real WhatsApp mentions (blue, tappable, resolved to each recipient's contact name) instead of plain "@Number" text.

How

• New optional field mentions: []string in SendMessageRequest (accepts phone numbers or full JIDs; numbers are normalized to <number>@s.whatsapp.net).
• When mentions are present, the text is sent as an ExtendedTextMessage with ContextInfo.MentionedJID set (this is the metadata official clients attach for mentions). Without mentions, behaviour is unchanged (plain Conversation message).

Usage

POST /api/send
{
  "recipient": "1234567890-123456@g.us",
  "message": "Welcome @31612345678!",
  "mentions": ["31612345678"]
}

The message text should include "@" for each mentioned user; WhatsApp clients render it using each viewer's own contact list.

Notes

• Backward compatible: mentions is optional, existing calls unchanged.
• Tested on a live bridge (direct chats and groups): mentions render blue and tappable, with contact-name resolution on the recipient side.

[EmilioNicolas]

Correction from live testing: the API field itself populates ContextInfo.MentionedJID, but clients do not replace visible @phone text with the sender contact name. Also, visible phone text paired with @lid metadata rendered as underlined phone numbers rather than named mentions. The PR description should not claim automatic contact-name substitution. I am investigating the exact payload used by official clients before recommending production use.