[web] Tickets and Clients as first-class workspace views

[e3ndr-agent]

The workspace treated the queue as the default and the clients page as an overlay: the topbar "clients" button only ever turned the view on, so once on the clients page there was no way back to tickets from the topbar (only the close button in the page header).

Turn Tickets and Clients into peer, first-class views:

• A "tickets" nav button (all staff) + the "clients" button (admin/superuser) now switch a view state ('tickets' | 'clients') instead of toggling a boolean.
• Either view is reachable at any time from the topbar; the queue's keyboard shortcuts (t /) are inert while the clients view is showing; opening a ticket from the bell returns to the tickets view.
• The clients page header's close button still returns to tickets (second exit).

No change to the content of either view — only how they're switched. This is the first step toward the larger nav revision (left-rail consolidation, issue #95).

Closes #102

Authored/Written by: Qwen3.8-27b