Header: hover-open dropdowns, per-item underline, Model dropdown

[PavelMakarchuk]

Reworks the desktop header to feel more responsive and to expose Model sections directly instead of bouncing users through the model overview.

Behaviour

• Dropdowns open on hover (About, Model) with a 100ms intent delay and a 200ms grace delay before close. A transparent paddingTop "bridge" inside the same hover container fills the 10px visual gap between trigger and panel, so the cursor doesn't trigger close while travelling to the panel.
• Click toggles still work for touch and keyboard. Escape and outside-click close the dropdown as before.
• Center-out underline grow on hover (white 2px bar, transform: scaleX(0→1)). The active page — or any child of a dropdown trigger — keeps the underline lit, so the bar always shows where you are.
• Focus = hover affordance so keyboard users get the same feedback as pointer users.

Model dropdown

• The flat Model → /model link expands into a sidebar mirror:
  • Rules — Coverage, Parameters, Variables
  • Data — Pipeline, Calibration, Validation
  • Behavioral responses
• Children are indented one level under their parent. Parents (Rules, Data, Behavioral responses) are clickable to their overview pages, which were just added in policyengine-model#41.
• All Model links use <a> via a new external flag on DropdownItem so the Vercel multizone rewrite is hit instead of a client-side route that the website zone can't fulfil.

Mobile

• MobileMenu now walks the nested children too and honors external, so Model sub-pages route correctly on touch devices.

Small things

• Logo→nav gap bumped from 12px to 40px so the logo reads as an anchor rather than another menu entry.
• Per-property transition delays on dropdown rows: the cascading entrance reveal staggers opacity only, while hover background/color stay instant. Fixes the visible lag when sweeping the cursor across rows of an open dropdown.
• CountrySelector hover inlined after removing the shared hoverHandlers helper (it no longer fit the underline pattern).

Test plan

• Hover any nav item → underline grows from center; leaving the item retracts it
• Hover About → dropdown opens after ~100ms; mousing down into a child via the gap does not close it
• Hover Model → same; children render indented with slightly muted color, parents clickable
• Sweep mouse rapidly across dropdown rows → hover background tracks the cursor with no perceptible delay (regression check for the per-property delay fix)
• Navigate to /us/research → Research underline stays lit
• Navigate to /us/model/rules/coverage → Model underline stays lit (active state walks dropdown children)
• Tab through nav with keyboard → each item lights up on focus; Escape closes an open dropdown
• Resize to mobile → menu icon opens slide-out; Model dropdown shows nested children indented; clicking a Model child crosses zones correctly
• Click anywhere outside an open dropdown → it closes

🤖 Generated with Claude Code

[vercel]

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Project	Deployment	Actions	Updated (UTC)
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