fix(panes): keep a pane window in step with theme + interface scale

The pane window got a ONE-TIME snapshot of the app's theme classes and the
interface-scale custom property. The app changes both at runtime — Interface
size emits `scale:changed`, the theme emits `theme:changed` /
`v3:cosmetics-applied` — so an already-open pane went on rendering at the old
scale, in the old palette, the moment the user touched either.

"Looks identical" has to keep being true, not merely start out true.

A pane window now follows those three events for as long as it is open, and
stops on unplace(). The inline style is assigned wholesale rather than merged:
unlike the class lists (where pane.html's own `fb-pane-window` must survive),
there is nothing in the pane document's inline style to preserve — and
concatenating on every change would grow the attribute without bound as the
user dragged the scale slider.

Also: the dock's focus() always smooth-scrolled, ignoring
prefers-reduced-motion — which panes.css already honours for the card's flash
animation. A smooth scroll is motion too, and someone who asked for less of it
meant this as well.

Signed-off-by: topkoa <topkoa@gmail.com>

static/panes/pane-dock.js

@@ -104,7 +104,11 @@
     function focus(id) {
         const card = cards.get(id);
         if (!card) return;
-        card.scrollIntoView({ block: 'nearest', behavior: 'smooth' });
+        // Honour prefers-reduced-motion, as the flash animation below already does
+        // in panes.css. A smooth scroll is motion too, and a user who asked for less
+        // of it meant this as well.
+        const calm = window.matchMedia && window.matchMedia('(prefers-reduced-motion: reduce)').matches;
+        card.scrollIntoView({ block: 'nearest', behavior: calm ? 'auto' : 'smooth' });
         // Re-trigger the flash even if the class is still there — repeat focus of
         // the same card would otherwise be a no-op animation.
         card.classList.remove('is-flash');