fix(popup): reduce width-adjustment flash in settings view

Two-pass scrollbar layout: .settings-content had overflow-y:auto which
triggered a scrollbar evaluation on every body width change, causing a
transient layout shift. Changed to overflow-y:hidden since the settings
panel never needs to scroll.

Compositor timing gap: even without the scrollbar evaluation, the browser's
1-2 frame lag between renderer and browser process popup window resize
remained visible. Added width:60ms transition to body.transitions-enabled
so each 10px step animates gradually (~2px/frame), making the timing gap
imperceptible.

Author: euxx

src/popup/popup.css

@@ -38,7 +38,10 @@ body.popup-ready {
 body.transitions-enabled {
   transition:
     background-color 0.2s ease-out,
-    color 0.2s ease-out;
+    color 0.2s ease-out,
+    /* Smooth popup-window resize to eliminate the 1-2 frame compositor timing
+       gap that produces a visible flash when body width changes. */
+    width 60ms ease-out;
 }
 
 body.transitions-enabled * {
@@ -1028,7 +1031,10 @@ body.dark-theme .settings-view {
 .settings-content {
   flex: 1;
   padding: 16px 20px;
-  overflow-y: auto;
+  /* Use hidden instead of auto: the settings panel never needs to scroll,
+     and overflow-y: auto triggers a two-pass scrollbar layout on every width
+     change, causing a ~17px horizontal flash on the right side of the popup. */
+  overflow-y: hidden;
 }
 
 .setting-row {