v11.3 Docs: Support full-width @sign for mentions in message composer

[Combs7th]

@claude - Update the Mattermost End User Guide to include a note in the mentions section:

• Describe that both the standard “@” and the full-width “＠” now trigger user mention suggestions.
• Add an example using Japanese IME where full-width ＠ triggers the mention list.
• Clarify backward compatibility: existing @ mentions continue to work as before.

Include cross-reference to release notes for v11.3.0.

[claude]

Claude finished @Combs7th's task —— View job • claude/issue-8668-20260121-0438 • Create PR ➔

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Working on v11.3 Docs: Support full-width @sign for mentions in message composer

Todo List:

• Read CLAUDE.md for repo-specific guidelines (no CLAUDE.md found)
• Find the End User Guide mentions section
• Locate v11.3.0 release notes for cross-reference
• Update mentions section with full-width @ support documentation
• Commit and push changes
• Provide PR creation link

Summary

Successfully updated the Mattermost End User Guide mentions section to document the new full-width @ symbol support introduced in v11.3. The documentation now includes:

• Full-width @ support: Both standard "@" and full-width "＠" (U+FF20) now trigger mention suggestions
• Japanese IME example: Specific example showing how Japanese input methods benefit from this feature
• Backward compatibility: Clear statement that existing @ mentions continue to work as before
• Cross-reference: Link to v11.3 release notes for additional details

The changes were committed and pushed to the branch. Ready for review!

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