Mobile layout pass: collapsible sidebar and stacked/scrollable tables

[alexeygrigorev]

Part of UX epic #67. From the operator UX audit (finding 14, LOW/MEDIUM).

Problem

On 390px the full sidebar + client switcher stack above all content, and wide tables (client list, campaign list, audience membership) overflow horizontally. Design system: prioritize core state/actions before large tables on mobile. (base.html:36-95)

Ask

Collapse the sidebar into a toggle on mobile; switch list tables to a stacked/card layout or a horizontal-scroll container with a visible affordance.

Acceptance

• Sidebar collapses behind a toggle on mobile.
• List tables no longer cut off horizontally.
• Tests + lint pass.