fix(desktop): resolve overlapping mention recipients by span

[yinkev]

Summary

• resolve outgoing mentions by their concrete text spans instead of independently accepting every candidate whose name is a prefix of the visible mention
• preserve an explicit autocomplete selection when its display name occurs once; when later edits create multiple occurrences, stop treating that name-only record as a global preference
• for pasted/manual ambiguous text, choose the longest exact display name within each overlapping span
• keep separate intentional short-name and long-name mentions independent, so both identities are emitted only when both spans are actually present
• move recipient extraction into a focused helper, keeping useMentions.ts below the repository's file-size limit

Before this change, @OriginalName copy matched both OriginalName and OriginalName copy, so Desktop emitted two p tags and could start two distinct managed agents even though the composer visibly named only the copy.

Related issue

Fixes #2909

No competing implementation was found before opening this PR. Related historical mention work addresses autocomplete or other routing mechanisms, not outgoing overlapping-span tag extraction.

Testing

• focused mention and outgoing-recipient regressions — 35 passed
• pnpm typecheck
• full Desktop test suite — passed
• pnpm build
• Biome format/check on all changed files
• git diff --check

The unrelated runtime file-size ratchet was subsequently restored upstream by the runtime.rs module split. After extracting the new recipient helper, useMentions.ts remains below its 1,000-line limit.

[yinkev]

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[yinkev]

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