bucket-bar: hide scrollbar, soft edge fades, wheel-to-scroll

The horizontal scrollbar on the bucket row was ugly and
inconsistent with the rest of the app's scrollbar-less panels. Hide
it, redirect vertical wheel over the bar to horizontal scroll, and
surface overflow with soft bg-coloured gradients on whichever side
has more content off-screen: right only while anchored at the
start, both in the middle, left only at the end.

Fade overlays live on #bucket-row (parent) as pseudo-elements with
pointer-events:none, rather than mask-image on the bar itself — a
mask would also clip super-bucket popovers that are intentionally
rendered inside #bucket-bar for the collab MutationObserver.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config.php

@@ -1,7 +1,7 @@
 <?php
 
 define('APP_NAME', 'hyprcrd');
-define('APP_VERSION', '1.9.6');
+define('APP_VERSION', '1.9.7');
 
 // Database path — writable directory, outside webroot
 // DB_PATH / UPLOAD_DIR can be pre-defined by the Electron router for app isolation