refactor(ui): add size variants to Switch and consolidate styles

Introduce sm/md/lg sizes on the Switch component driven by CSS custom
properties, move switch styling into a dedicated switch.css, and remove
the duplicated overrides from styles.css and ToolbarCustomizeDialog.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/styles.css

@@ -292,33 +292,6 @@ button:disabled {
   cursor: pointer;
 }
 
-.ui-switch {
-  position: relative;
-  width: 46px;
-  height: 26px;
-  border: 0;
-  border-radius: 999px;
-  background: #d7dce4;
-}
-
-.ui-switch[data-state="checked"] {
-  background: #303846;
-}
-
-.ui-switch-thumb {
-  display: block;
-  width: 18px;
-  height: 18px;
-  transform: translateX(4px);
-  border-radius: 999px;
-  background: #fff;
-  transition: transform 0.18s ease;
-}
-
-.ui-switch[data-state="checked"] .ui-switch-thumb {
-  transform: translateX(24px);
-}
-
 .model-tabs {
   display: grid;
   grid-template-columns: repeat(3, 1fr);