Add resizable room panel, searchable user picker, and direct room membership

Room info panel is now user-resizable (fixing a layout clip at narrow
widths), and every user-selection spot (room membership, admin ownership
transfer) uses a new searchable UserPicker instead of raw text input or
prompt(). Member rows fold role + actions into a single inline dropdown
instead of a row of buttons, so the member list stays usable as rooms grow.

Room invites (the accept/decline flow) are replaced by adding a user to a
room directly -- an admin/owner picks someone and they're a member
immediately, with a "you've been added" notification email instead of an
invite email. Drops the now-unused room_invites table.

backend/app/models/site_invite.py

@@ -16,7 +16,7 @@ def _default_expires_at() -> datetime:
 
 class SiteInvite(Base):
     """An admin-issued invite for someone with no account yet -- distinct
-    from RoomInvite, which targets an existing user for a specific room."""
+    from adding an existing user directly to a room."""
 
     __tablename__ = "site_invites"