fix: remove default mt-2 margin from TabsContent

Removes the default mt-2 margin from TabsContent component to fix
inconsistent tab content heights in spell viewers. All spell tab
implementations explicitly use m-0, and this default margin was
causing layout calculation issues when switching between tabs.

Affected components:
- ProfileViewer: spell tabs now render at consistent heights
- EventDetailViewer: spell tabs now render at consistent heights
- RelayViewer: spell tabs now render at consistent heights

Other usages (SettingsViewer, SettingsDialog) explicitly use m-0
and are unaffected. LoginDialog tabs will have slightly tighter
spacing but remain functional.

src/components/ui/tabs.tsx

@@ -42,7 +42,7 @@ const TabsContent = React.forwardRef<
   <TabsPrimitive.Content
     ref={ref}
     className={cn(
-      "mt-2 ring-offset-background focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2",
+      "ring-offset-background focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2",
       className,
     )}
     {...props}