fix: unify day boundaries on device-local time, JSON import preview parity

- Limits/streaks/greeting now use device-local days (was hardcoded Europe/London),
  matching dashboard metrics; label copy updated
- JSON imports now go through the same preview/duplicate-detection modal as Excel

src/components/ImportPreviewModal.tsx

@@ -37,7 +37,9 @@ export function ImportPreviewModal({
         >
           <div className="mb-5 flex items-start justify-between gap-4">
             <div>
-              <p className="text-sm font-medium uppercase tracking-[0.18em] text-pink-100">Excel import</p>
+              <p className="text-sm font-medium uppercase tracking-[0.18em] text-pink-100">
+                {preview.fileName.toLowerCase().endsWith(".json") ? "JSON import" : "Excel import"}
+              </p>
               <h2 id="import-preview-title" className="mt-1 text-3xl font-semibold tracking-tight text-white">
                 Preview rows
               </h2>