fix: use blob URL for Firefox downloads (data: URLs are access-denied)

Firefox rejects data: URLs in browser.downloads.download with
"Access denied for URL data:...", breaking the PDF export. Build the
download URL per-browser: a blob: object URL where URL.createObjectURL
exists (Firefox background page), falling back to the base64 data: URL
for Chrome MV3 service workers (no createObjectURL).

Bump version to 1.3.0.

manifest.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "ISMIS Helper",
-  "version": "1.2.0",
+  "version": "1.3.0",
   "description": "Auto-login, auto-accept privacy policy, and Study Load PDF for the USC ISMIS portal.",
   "permissions": ["storage", "downloads"],
   "host_permissions": ["https://ismis.usc.edu.ph/*"],