fix(#931): replace mojibake (corrupted UTF-8) strings in useStudyGroups toast and notification messages

Fix three user-facing strings in useStudyGroups.tsx that showed garbled characters:

- toast.success: 'Created group "..."' - smart quotes were corrupted to '“/â€Â�'\n- addNotification toast: same quote corruption in message template\n- toast 'Left group': wave emoji icon corrupted from '👋' to '👋'\n\nThese were caused by UTF-8 bytes being misinterpreted as Latin-1 (mojibake). Replaced with proper ASCII quotes and the Unicode emoji character.

Author: iyanumajekodunmi756

src/app/hooks/useStudyGroups.tsx

@@ -265,11 +265,11 @@ export function useStudyGroups(currentUser?: { id: string; name: string }): UseS
         const { addNotification } = useNotificationStore.getState();
         addNotification({
           type: 'success',
-          message: `Created group “${group.name}”`,
+          message: `Created group "${group.name}"`,
           meta: { groupId: group.id },
         });
       } catch {}
-      toast.success(`Created group “${group.name}”`);
+      toast.success(`Created group "${group.name}"`);
       return group;
     },
     [me.id, me.name, triggerSync],
@@ -326,7 +326,7 @@ export function useStudyGroups(currentUser?: { id: string; name: string }): UseS
           meta: { groupId },
         });
       } catch {}
-      toast('Left group', { icon: '👋' });
+      toast('Left group', { icon: '👋' });
     },
     [me.id, triggerSync],
   );