feat: implement JSON backup export functionality with improved download handling

Co-authored-by: Copilot <copilot@github.com>

wwwroot/theme.js

@@ -86,6 +86,39 @@ window.workTrackerPreferences = {
     }
 };
 
+window.workTrackerDownloads = {
+    async downloadFromUrl(url) {
+        const response = await fetch(url, {
+            credentials: "same-origin"
+        });
+
+        if (!response.ok) {
+            throw new Error(`Download failed with status ${response.status}`);
+        }
+
+        const blob = await response.blob();
+        const contentDisposition = response.headers.get("content-disposition") || "";
+        const fileNameMatch = /filename\*=UTF-8''([^;]+)|filename="?([^";]+)"?/i.exec(contentDisposition);
+        const fileName = decodeURIComponent(fileNameMatch?.[1] || fileNameMatch?.[2] || "worktracker-backup.json");
+
+        const objectUrl = URL.createObjectURL(blob);
+
+        try {
+            const anchor = document.createElement("a");
+            anchor.href = objectUrl;
+            anchor.download = fileName;
+            anchor.rel = "noopener";
+            anchor.style.display = "none";
+            document.body.appendChild(anchor);
+            anchor.click();
+            anchor.remove();
+        }
+        finally {
+            URL.revokeObjectURL(objectUrl);
+        }
+    }
+};
+
 window.workTrackerDateInput = (() => {
     const listeners = new WeakMap();