Improved @mentions

@mention dropdown now groups users by assignment group (e.g. Basic Support, Member Services) so agents can quickly find the right team member.

Conversations on mobile: reply and internal note inputs stay in view without horizontal scrolling; content fits the screen width.

@mention autocomplete now matches full names with spaces (e.g. @First Last).

Inbound emails now match clients for auto-assignment regardless of email capitalization.

Creating or updating a site with an address that already exists now shows a clear message instead of a technical error.

Sidekick policies filtered by Site Label or Service Level now show as Applicable when viewing a site.