Remove drop shadows from composer and user message bubbles

Strip shadow-composer (and its focus/open-state variants) from the
composer surface, composer fallback surface, and the shared user-bubble
base class. Also drop the !important box-shadow override on
[data-slot=composer-surface] that re-applied the shadow regardless of
the utility class, so the flatter look actually takes effect.

apps/desktop/src/styles.css

@@ -943,7 +943,6 @@ canvas {
 
 [data-slot='composer-surface'] {
   border-color: var(--ui-stroke-secondary) !important;
-  box-shadow: var(--shadow-composer) !important;
 }
 
 [data-slot='composer-fade'] {