Support inbox: full-height collapsible filter rail (Linear-style)

UX audit finding — implemented

The support desk stacked ~220px of header chrome (title, description, 3 filter rows) before the ticket list. Replaced with a full-height collapsible InboxRail (264px expanded / 64px icon-only) holding Views, Queues, Status, New ticket, and Settings.

List + conversation now use the full canvas height; mobile keeps horizontal filter chips.