feat(css): cross-document view transitions

@view-transition { navigation: auto; } lets Chrome 126+ / Safari 18+
crossfade same-origin navigations — sidebar links and the series
page's library-switch navigation get a subtle fade with zero
JavaScript, softening the switch to full SSR navigation. Pure
progressive enhancement: Firefox and older browsers simply navigate
instantly. style.css regenerated via build:css:prod.

web/static/input.css

@@ -3,6 +3,18 @@
 @tailwind components;
 @tailwind utilities;
 
+/*
+ * Cross-document view transitions (the optional sweetener): modern
+ * browsers (Chrome 126+, Safari 18+) crossfade same-origin navigations —
+ * sidebar links and the series page's library-switch navigation get a
+ * subtle fade for free, with zero JavaScript. Firefox and older browsers
+ * simply navigate instantly; the rule is pure progressive enhancement.
+ */
+@view-transition {
+  navigation: auto;
+}
+
+
 @layer base {
   /*
    * Default :root mirrors the Tokyo Night theme so the palette is correct