Docked Assistant Panel & Permissions UI

Docked Assistant panel with Dock/Undock toggle (restores user preference on reload).

User permissions UI now alphabetizes selected permissions for easier scanning.

Missing "merged conversation" message when merging into a conversation with no prior user-written messages.

Labels could be selected multiple times before backend response — now prevented.

Creating a new site + client during conversation creation no longer shows undefined in the Clients field.