fix(desktop): preserve exact agent mentions on send

[alexesms]

Summary

• preserve the Nostr p tag when a user clicks Send with an exact agent mention still pending as plain autocomplete text
• resolve only uniquely named agents, so duplicate display names are never guessed
• cover trailing mentions, mentions at the beginning, and punctuation boundaries
• move send-time extraction into a focused helper to keep useMentions.ts under the repository file-size limit

Related issue

N/A — searched existing issues and pull requests. #2508, #2514, and #2893 cover cross-machine agent discovery; this fixes the separate send-time path where a known agent is visible but the autocomplete suggestion has not been selected before Send is clicked.

Testing

• desktop unit suite: 3,723 passed
• mention helper unit tests: 29 passed
• Playwright mention suite: 50 passed
• focused Playwright regression: 1 passed
• pnpm typecheck
• Biome check for changed files
• desktop file-size checks
• just ci passed workspace fmt/clippy, desktop checks, Tauri fmt/clippy, and web checks
• just ci mobile step did not complete because Hermit's Dart bootstrap stalled without output for more than eight minutes; no mobile files are changed