swarm-ui: page-switch animation (pop entrance + nav underline travel)

Shell's page content now remounts on every navigation and plays a
scale+fade pop-in entrance. The active nav link's underline is now a
single shared element that slides to its new position instead of
snapping, re-colouring to a per-tab accent — a discrete cycle through
the existing base16 chromatic slots, not a continuous hue rotation.
Both are gated on the data-motion/prefers-reduced-motion plumbing
lib/motion-apply.ts already had wired and waiting for a first real
consumer.

frontend/packages/swarm-ui/src/shell/Shell.tsx

@@ -12,20 +12,30 @@
 // has exactly one place that needs to know its own nav, and this is
 // it. Grows additively as real routes land (a swarm-wide agent roster
 // is next); no speculative entries.
-import { useEffect, useState } from 'preact/hooks';
+//
+// Page-switch animation, two pieces (motion-guard rules live in
+// Shell.css): `.shell-body` remounts on every navigation
+// (`key={location}`) to replay a pop-in entrance, and a single shared
+// underline (`.shell-nav-indicator`, replacing what used to be a
+// per-link border) slides to the active link's measured position and
+// re-colours to that tab's own accent. Accent is a discrete cycle
+// through the existing base16 chromatic slots (`NAV_ITEMS`' `accent`
+// field), not a continuous hue rotation — everything here still
+// derives from base16, same rule the rest of the palette follows.
+import { useEffect, useRef, useState } from 'preact/hooks';
 import type { ComponentChildren } from 'preact';
-import { Link, useRoute } from 'wouter-preact';
+import { Link, useLocation } from 'wouter-preact';
 import { LinksMenu } from './LinksMenu.js';
 import { SettingsMenu } from './SettingsMenu.js';
 import { useApplyThemeOverride } from '../lib/theme-apply.js';
 import { useApplyMotionOverride } from '../lib/motion-apply.js';
 import './Shell.css';
 
-const NAV_ITEMS: { href: string; label: string }[] = [
-  { href: '/', label: 'hives' },
-  { href: '/create-agent', label: 'new agent' },
-  { href: '/jobs', label: 'jobs' },
-  { href: '/components', label: 'components' },
+const NAV_ITEMS: { href: string; label: string; accent: string }[] = [
+  { href: '/', label: 'hives', accent: 'var(--purple)' },
+  { href: '/create-agent', label: 'new agent', accent: 'var(--cyan)' },
+  { href: '/jobs', label: 'jobs', accent: 'var(--pink)' },
+  { href: '/components', label: 'components', accent: 'var(--blue)' },
 ];
 
 // Static fallback — matches `index.html`'s `<title>` default, so a page
@@ -34,21 +44,47 @@ const NAV_ITEMS: { href: string; label: string }[] = [
 // same generic label the pre-fetch page already showed, not a blank.
 const DEFAULT_BRAND = 'hyperhive swarm';
 
-function NavLink({ href, label }: { href: string; label: string }) {
-  const [active] = useRoute(href);
+interface IndicatorRect {
+  left: number;
+  top: number;
+  width: number;
+  accent: string;
+}
+
+function NavLink({
+  href,
+  label,
+  active,
+  textRef,
+}: {
+  href: string;
+  label: string;
+  active: boolean;
+  textRef: (el: HTMLSpanElement | null) => void;
+}) {
   return (
     <Link href={href} className="shell-nav-link">
       {/* Touch target (min-height) lives on the <a> so the whole row is
-          tappable; the active-state underline lives on this inner span
-          so it hugs the text instead of sitting at the bottom of the
-          full-height box, ~0.7em away from it. */}
-      <span class={'shell-nav-link-text' + (active ? ' shell-nav-link-active' : '')}>{label}</span>
+          tappable. The underline itself no longer lives here — see
+          `.shell-nav-indicator` — this span is still what gets
+          measured to position that shared indicator, so it hugs just
+          the text rather than the full-height tappable box. */}
+      <span
+        ref={textRef}
+        class={'shell-nav-link-text' + (active ? ' shell-nav-link-active' : '')}
+      >
+        {label}
+      </span>
     </Link>
   );
 }
 
 export function Shell({ children }: { children: ComponentChildren }) {
   const [swarmName, setSwarmName] = useState<string | null>(null);
+  const [location] = useLocation();
+  const navRef = useRef<HTMLElement | null>(null);
+  const textRefs = useRef<Record<string, HTMLSpanElement | null>>({});
+  const [indicator, setIndicator] = useState<IndicatorRect | null>(null);
 
   // Applied once here, not inside `SettingsMenu` — every route mounts
   // through this one `<Shell>`, so the override takes effect regardless
@@ -59,6 +95,36 @@ export function Shell({ children }: { children: ComponentChildren }) {
   useApplyThemeOverride();
   useApplyMotionOverride();
 
+  // Re-measures on every navigation and on resize (the nav's own
+  // `flex-wrap` means a link's position genuinely changes at narrow
+  // widths, not just its route). `getBoundingClientRect()` on both
+  // elements at the same tick keeps this scroll-position-agnostic — a
+  // difference of two viewport-relative rects is scroll-invariant, no
+  // separate scroll-offset bookkeeping needed. No indicator (hidden via
+  // `left: 0; width: 0`) on a route with no matching nav item (404).
+  useEffect(() => {
+    function measure() {
+      const navEl = navRef.current;
+      const activeItem = NAV_ITEMS.find((item) => item.href === location);
+      const textEl = activeItem ? textRefs.current[activeItem.href] : null;
+      if (!navEl || !activeItem || !textEl) {
+        setIndicator(null);
+        return;
+      }
+      const navRect = navEl.getBoundingClientRect();
+      const textRect = textEl.getBoundingClientRect();
+      setIndicator({
+        left: textRect.left - navRect.left,
+        top: textRect.bottom - navRect.top,
+        width: textRect.width,
+        accent: activeItem.accent,
+      });
+    }
+    measure();
+    window.addEventListener('resize', measure);
+    return () => window.removeEventListener('resize', measure);
+  }, [location]);
+
   // Fetched once here, not per-page: every route mounts inside one
   // `<Shell>`, and the swarm's name doesn't change within a page
   // visit. A fetch failure is silently ignored — `swarmName` just stays
@@ -85,10 +151,31 @@ export function Shell({ children }: { children: ComponentChildren }) {
     <div class="shell">
       <header class="shell-header">
         <span class="shell-brand">{brand}</span>
-        <nav class="shell-nav">
+        <nav class="shell-nav" ref={navRef}>
           {NAV_ITEMS.map((item) => (
-            <NavLink key={item.href} href={item.href} label={item.label} />
+            <NavLink
+              key={item.href}
+              href={item.href}
+              label={item.label}
+              active={item.href === location}
+              textRef={(el) => {
+                textRefs.current[item.href] = el;
+              }}
+            />
           ))}
+          {/* Shared sliding underline — see the file-top comment. Hidden
+              (zero width) rather than unmounted when there's no active
+              nav item (404), so it doesn't pop in with a stale position
+              the next time a real nav item becomes active. */}
+          <span
+            class="shell-nav-indicator"
+            style={{
+              left: `${indicator?.left ?? 0}px`,
+              top: `${indicator?.top ?? 0}px`,
+              width: `${indicator?.width ?? 0}px`,
+              backgroundColor: indicator?.accent ?? 'transparent',
+            }}
+          />
         </nav>
         {/* Single `margin-left: auto` on the wrapper, not on each menu
             individually — two adjacent flex items both set to
@@ -99,7 +186,11 @@ export function Shell({ children }: { children: ComponentChildren }) {
           <LinksMenu />
         </div>
       </header>
-      <div class="shell-body">{children}</div>
+      {/* Keyed by route so it remounts (and replays its entrance
+          animation) on every navigation — see the file-top comment. */}
+      <div class="shell-body" key={location}>
+        {children}
+      </div>
     </div>
   );
 }