feat(desktop): live @mention and #channel preview in message composer

[baxen]

Summary

Adds a transparent overlay to the message composer that renders @mention and #channel tokens with styled highlight pills as the user types.

Changes

• ComposerMentionOverlay.tsx (new) — Parses composer text into segments (plain text, @mention, #channel) and renders mentions/channels with colored pill styling (bg-primary/15 text-primary).
• MessageComposer.tsx — Wraps the textarea in a relative container with the overlay positioned behind it. The textarea text is made transparent (text-transparent) so the overlay's styled tokens show through, while the native caret, selection, scroll, and IME behavior are fully preserved.

How it works

1. The textarea renders with text-transparent and caret-foreground so the cursor is visible but the raw text is not.
2. An aria-hidden overlay sits behind the textarea and mirrors the content with highlighted pills for @ and # tokens.
3. Scroll position is synced via an onScroll handler so the overlay stays aligned during multi-line input.

Testing

• Verified visually that mentions and channels highlight in real time as you type.
• Confirmed cursor, selection, paste, and IME input work normally.
• All CI checks pass (fmt, clippy, unit tests, desktop build, tauri check).

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Match mention tokens that include spaced display names

useMentions.insertMention builds mentions as @${displayName} and displayName values commonly include spaces, but this regex uses @\S+, which stops at the first space. As a result, a mention like @Jane Doe is rendered as a partial pill (@Jane) plus plain text (Doe), so the new live preview is inconsistent with the actual mention semantics (extractMentionPubkeys still treats the full display name as one mention) and gives misleading feedback while composing.

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