Umbrella: finish overlay primitives after #2227

[zachdunn]

Tracking leftover overlay/filter work after #2227 (Base UI popover, select, dialog on search + settings).

#2227 shipped the overlay layer and used it on the surfaces that were hurting. These follow-ups are the rest of the homemade menus and the next search-filter step. Look stays in @releases/design-system; overlay behavior stays in web/src/components/ui. Do not grow the design-system package into an in-house Radix.

Done

• Popover / Select / Dialog restyled onto house tokens (web/src/components/ui)
• Search bar + org/source ReleaseFilterInput + collection timeline filters on Popover
• Settings Select (webhook format/type) + ConfirmDialog (revoke/delete/rotate)
• Deduplicate local inputClass copies onto DS / auth-flow constants
• Going-forward rule in packages/design-system/README.md: no new pointerdown + Escape menus

Remaining

• DropdownMenu for leftover homemade menus (same pointerdown + Escape copy in each):
  • web/src/components/open-in-agent-menu.tsx
  • web/src/components/release-admin-menu.tsx
  • web/src/components/source-admin-menu.tsx
  • web/src/components/collection-admin-menu.tsx
  • web/src/components/report-issue.tsx
  • web/src/components/account-nav.tsx (header account menu)
  • web/src/components/account-settings-nav.tsx (workspace selector)
  • web/src/components/org/agent-copy-button.tsx (split-button Copy for agent)
• HoverCard — replace homemade web/src/components/hover-card.tsx with Base UI PreviewCard or Tooltip (pick one, not both). Callers: date-tooltip, info-tooltip, importance-marker, cadence-chrome, org-table, range-navigator. Do not grow hover-card.tsx.
• Combobox when search grows a typeahead facet (org / tag / product). Do not invent one in ReleaseFilterInput.
• Sheet for SourceWorkflowDrawer (homemade right-hand overlay) when that file is next touched.
• Auth chrome (auth-flow.tsx) still has a slightly taller/rounder button + input than settings DS. Unify only if we want one action language site-wide.

Not this

Leave these homemade. They are product/chrome UI, not missing overlay primitives:

• Lightbox (lightbox.tsx) — media viewer with paging
• Mobile nav (mobile-nav.tsx) — one header caller, not a Sheet
• Range navigator brush/timeline — product viz; nested quick-ranges can ride Select when that file is next edited
• Native <select> (changelog file picker, fetch-plan priority, workspace invitations)
• Link tabs (SourceTabs, org filter tabs) — routing, not Base UI Tabs
• DS Toggle / ThemeToggle / buttons / inputs — look, already in @releases/design-system

Add a child issue per item when it starts, rather than stuffing implementation notes here.