#182 fix(ui): collapse the top nav into a drawer on mobile and tablet screens (#154)

[Anonymous]

Originally created by: ddlaurin

Closes [#154]

Problem

The header is a single flex row whose children default to min-width: auto, so
nothing could shrink below its content width. Platform view needs ~1079px to lay
its seven tabs out inline, so a 375px viewport forced the document to 962px and
the whole page scrolled sideways.

Change

• Tabs collapse into a Sheet drawer below lg (1024px), covering phones and tablets
• Header spacing tightens between lg and xl so all seven tabs fit at 1024px
• Drawer open state is derived from the current route, so back/forward, the org
  switcher and programmatic navigation all close it
• Brand wordmark and the "Hire agent" label collapse below sm

Checks

• pnpm exec tsc --noEmit clean
• pnpm lint clean
• pnpm exec playwright test 251 passed (including 8 new specs)

[Anonymous]

Ticket changed by: ddlaurin

• status: open --> closed