fix(mobile): tag DM recipients

[ik24999]

Summary

• tag every other participant as a recipient when sending a mobile direct message
• keep channel messages and typing streams limited to explicit mentions
• add regression tests for direct-message recipients and stream events

Root cause

Mobile sends only encoded explicit mentions as p tags. In a direct message that means an ordinary message can be stored without any recipient tag, so managed agents never receive it and clients cannot treat it as addressed to them.

Desktop already derives DM recipients from the channel membership. This change brings mobile in line with that behavior while excluding the sender and deduplicating recipients.

User impact

Direct messages sent from Buzz mobile will reliably notify and wake their recipients without requiring users to type an explicit @mention.

Verification

Validated after rebasing onto current block/buzz:main with Flutter 3.41.7 / Dart 3.11.5:

• dart format: 366 files, 0 changes
• flutter analyze: no issues
• node scripts/check-file-sizes.mjs: passed
• full flutter test: 1,231 tests passed
• git diff --check: passed
• remote branch tree 5cb4a66f24606afa7c6406ed8d52bc3f976d3ffb matches the locally validated tree

Originating Buzz channel: c3e004a6-08b4-4cd5-9c64-f84f4e599b4f