fix(ui,preview): panels chase the keyboard by frame, and the preview is finally mobile

The panel measured itself once, a fixed delay into the keyboard's animation,
so it landed against a viewport that was still moving - and in a sidedock
drawer no resize event fires at all, so it never moved. It now re-places every
frame until its own position holds still, which needs neither a duration to
guess nor an event to wait for. Its input taking focus is another trigger:
that is what opens the keyboard in the first place.

The preview had no viewport meta, so a 390px window still laid out at 980px:
every "mobile" screenshot was a desktop one at a phone's aspect, and
coarse-pointer rules could not apply. Which is why the tap targets looked
fine here and were not.

preview/index.html

@@ -2,6 +2,9 @@
 <html lang="en">
   <head>
     <meta charset="utf-8" />
+    <!-- Without this a phone-sized window still lays out at 980px, so a
+         "mobile" screenshot is not one and coarse-pointer rules never apply. -->
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
     <title>beaver-calendar preview</title>
     <link rel="stylesheet" href="/app.css" />
     <link rel="stylesheet" href="/theme.css" />