site: page transitions and interaction feedback

[AviOfLagos]

Adds cross-document view transitions and a set of small interaction
animations to the field-guide site. Builds on the multi-page split in #33.

Transitions

@view-transition { navigation: auto } lives in the shared stylesheet, so
every page opts in on both sides of a navigation. The nav and footer carry
view-transition-name, which lifts them out of the root snapshot — the
chrome holds still while the content cross-fades and rises, instead of the
whole viewport blinking.

<link rel="expect" href="#main" blocking="render"> holds the first paint
until <main> is parsed, so a transition never animates toward a
half-built page.

Micro-interactions

• Nav underline grows from the left rather than appearing
• Pressable things dip 1px on :active
• Pager arrows nudge toward their direction
• Copy button swaps to a tick that draws itself in
• Theme icon turns on hover
• Tab panels fade in instead of snapping
• Hamburger folds into a cross

Accessibility and support

All of it is a progressive enhancement. Browsers without cross-document
view transitions (currently Firefox) navigate normally.
prefers-reduced-motion: reduce sets navigation: none and drops every
animation, while keeping the states that carry meaning — the current-page
marker and the open-menu cross.

The underline is scoped to min-width: 861px; below that the drawer marks
the current page with a dot, and a full-width bar would fight it.

Verification

Verified structurally in-browser: both @view-transition rules parse into
the CSSOM (navigation: auto at top level, navigation: none inside the
reduced-motion query), onpagereveal support is present, the transition
names compute on nav and footer, and the hamburger's target transform
resolves correctly.

Not verified: the animations actually playing. The automation browser
reports visibilityState: "hidden", which suppresses CSS transitions and
causes view transitions to be skipped outright — so these need eyeballing
in a real browser before merge.

Build is deterministic and the internal link check passes (9 pages, 0 broken).