fix(file-manager): use OKLCH token directly for the multi-drag badge

The multi-drag "N items" drag image set `hsl(var(--primary))`, but the theme
tokens are OKLCH values — `hsl(oklch(...))` is invalid CSS and dropped, leaving
a transparent badge with default text. Reference the tokens directly via
`var(--primary)` / `var(--primary-foreground)` (theme-aware, valid).

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

frontend/src/components/shared/file-manager/use-file-manager-dnd.ts

@@ -290,8 +290,8 @@ export function useFileManagerDnd({
                     'top: -1000px',
                     'left: -1000px',
                     'padding: 6px 12px',
-                    'background: hsl(var(--primary))',
-                    'color: hsl(var(--primary-foreground))',
+                    'background: var(--primary)',
+                    'color: var(--primary-foreground)',
                     'border-radius: 6px',
                     'font: 500 13px system-ui, -apple-system, "Segoe UI", sans-serif',
                     'white-space: nowrap',