feat: responsive mobile improvements for staff portal

[gaghan430]

Summary

Comprehensive responsive/mobile improvements for the staff portal, addressing layout issues on viewports below 1024px.

New reusable components:

• useBreakpoint hook — mobile (<768px) / tablet (768–1023px) / desktop (≥1024px), SSR-safe, ported from cloud-portal
• SubLayout tab bar — replaces fixed sidebar with horizontally scrollable tabs on mobile/tablet, with specificity-based active state via shared pickMostSpecificHref helper
• DataTableToolbar — search goes full-width on mobile, filters collapse into a bottom Sheet with active-filter badge dot (rolled out to 24 list pages)
• PageHeader — title + description + actions, stacks vertically on mobile (rolled out to 6 detail pages)
• DescriptionList — semantic dl/dt/dd with CSS grid, stacks label above value on mobile (replaces Table-based key/value layouts on 5 detail pages)
• ActionCard — callout banner (warning/success/destructive/info) that stacks on mobile
• MobileToolbar — compact breadcrumb (back + current label) with overflow menu for trail + actions
• AppSearchMobile — icon trigger that opens a top Sheet with full-screen search

Bug fixes:

• App sidebar mobile: force Collapsible path instead of HoverCard (was causing cut-off flyout + auto-redirect on touch)
• SubLayout tabs: fix Overview always showing active on child pages (missing specificity logic)

Closes #406
Part of datum-cloud/enhancements#688

Test plan

• Resize between 390px / 800px / 1280px on all major flows
• Sidebar: hamburger → drawer → tap "Customers" → expands inline → tap child → navigates + closes
• SubLayout tabs: visit user/org/project detail pages → only current tab highlighted, not Overview
• Search: mobile icon → top Sheet → type query → results → navigate → sheet closes
• Toolbar: breadcrumb shows back + current on mobile, overflow menu has trail + actions
• DataTable: filter icon + bottom Sheet on pages with filters (users, organizations, fraud, email, activity)
• PageHeader: title + actions stack on mobile (user detail with Approve/Reject buttons)
• DescriptionList: label above value on mobile, 2-column grid on desktop (user/org/project/domain/export-policy detail)
• ActionCard: Deactivate/Reactivate/Delete banners stack on mobile (user detail + all DangerZoneCard pages)
• Desktop: all pages visually unchanged

Monosnap.screencast.2026-04-16.17-03-43.mp4

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[cla-assistant]

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[mattdjenkinson]

@gaghan430 nice work! Could you take a look at the dashboard as well please?

[gaghan430]

@mattdjenkinson I just did