fix(ui): theme popover/input tokens and open select below trigger

Select dropdowns (voice tab, guild selector) rendered with a transparent
background because --color-popover/--color-input were undefined, and the
popup overlapped the trigger due to alignItemWithTrigger. Define the missing
theme tokens (popover, popover-foreground, input, secondary) and default the
select popup to open below the trigger.

services/frontend/src/components/ui/select.tsx

@@ -63,7 +63,7 @@ function SelectContent({
   sideOffset = 4,
   align = "center",
   alignOffset = 0,
-  alignItemWithTrigger = true,
+  alignItemWithTrigger = false,
   ...props
 }: SelectPrimitive.Popup.Props &
   Pick<