fix: bake standard tooltip styling into TooltipContent component

Added font-mono tabular-nums text-stat-value directly to the
TooltipContent base styles so every tooltip in the app inherits
the same look without per-instance wrappers. Removed the now-
redundant span from StackRow's RowTooltip.

frontend/src/components/sidebar/StackRow.tsx

@@ -38,7 +38,7 @@ function RowTooltip({ trigger, label }: { trigger: ReactNode; label: string }) {
       <Tooltip>
         <TooltipTrigger asChild>{trigger}</TooltipTrigger>
         <TooltipContent side="bottom" sideOffset={4} align="center">
-          <span className="font-mono text-xs tabular-nums text-stat-value">{label}</span>
+          {label}
         </TooltipContent>
       </Tooltip>
     </TooltipProvider>

frontend/src/components/sidebar/__tests__/StackRow.test.tsx

@@ -43,7 +43,7 @@ describe('StackRow', () => {
   });
 
   it('wraps the partial pill in a hover tooltip', () => {
-    const { container } = render(<StackRow {...base({ status: 'partial', running: 3, total: 5 })} />);
+    render(<StackRow {...base({ status: 'partial', running: 3, total: 5 })} />);
     const trigger = screen.getByText('PT');
     // Radix TooltipTrigger adds data-state to the wrapped element.
     expect(trigger.getAttribute('data-state')).toBe('closed');