fix: make dropdown menu content scroll when taller than viewport

DropdownMenuContent (and SubContent) had overflow-hidden with no
max-height, so a dropdown with more items than fit the viewport —
e.g. the project selector in the New Plan dialog with many projects —
clipped the lower items with no way to reach them. Cap the height to
Radix's --radix-dropdown-menu-content-available-height and switch to
overflow-y-auto so the list scrolls.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dcrjodle

src/frontend/src/components/ui/dropdown-menu.tsx

@@ -63,7 +63,7 @@ const DropdownMenuSubContent = React.forwardRef<
     ref={ref}
     collisionPadding={8}
     className={cn(
-      "z-50 min-w-[8rem] max-w-[calc(100vw-16px)] overflow-hidden rounded-box border bg-popover p-1 text-popover-foreground shadow-lg flex flex-col gap-0.5 data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0 data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-95 data-[side=bottom]:slide-in-from-top-2 data-[side=left]:slide-in-from-right-2 data-[side=right]:slide-in-from-left-2 data-[side=top]:slide-in-from-bottom-2",
+      "z-50 min-w-[8rem] max-w-[calc(100vw-16px)] max-h-[var(--radix-dropdown-menu-content-available-height)] overflow-y-auto rounded-box border bg-popover p-1 text-popover-foreground shadow-lg flex flex-col gap-0.5 data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0 data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-95 data-[side=bottom]:slide-in-from-top-2 data-[side=left]:slide-in-from-right-2 data-[side=right]:slide-in-from-left-2 data-[side=top]:slide-in-from-bottom-2",
       className,
     )}
     {...props}
@@ -80,7 +80,7 @@ const DropdownMenuContent = React.forwardRef<
       ref={ref}
       collisionPadding={8}
       className={cn(
-        "z-50 min-w-[8rem] max-w-[calc(100vw-16px)] overflow-hidden rounded-box border bg-popover p-1 text-popover-foreground shadow-md flex flex-col gap-0.5",
+        "z-50 min-w-[8rem] max-w-[calc(100vw-16px)] max-h-[var(--radix-dropdown-menu-content-available-height)] overflow-y-auto rounded-box border bg-popover p-1 text-popover-foreground shadow-md flex flex-col gap-0.5",
         "data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0 data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-95 data-[side=bottom]:slide-in-from-top-2 data-[side=left]:slide-in-from-right-2 data-[side=right]:slide-in-from-left-2 data-[side=top]:slide-in-from-bottom-2",
         className,
       )}