feat(v1.4.0): CSV/Excel/JSON/NextUp exports + hybrid smart year standing

- Export modules (export-csv/json/xlsx) mirroring the PDF layout, plus a
  Next Up import CSV matching the raycast/nextup column contract.
- Hybrid Year Level: View Grades history (exact per-semester standing),
  falling back to units math (prospectus total - lacking), then profile.
  Popup adds a year-mode setting (auto/custom/off).
- On-page button row now offers PDF / CSV / Excel / JSON / Next Up.
- Bump version to 1.4.0.
- Tag-only build workflow with an optional AMO sign step for a signed .xpi;
  separate ci.yml keeps tests on pushes/PRs.
- Tests: year-standing parser tests + export tests (CSV quoting, JSON schema,
  XLSX structure); CI validates XLSX via openpyxl.

package.json

@@ -5,9 +5,11 @@
   "description": "Firefox extension for the USC ISMIS portal",
   "scripts": {
     "test": "node --test",
-    "build": "node scripts/build.js all"
+    "build": "node scripts/build.js all",
+    "sign": "web-ext sign --source-dir dist/firefox --channel unlisted --artifacts-dir dist --api-key \"$AMO_JWT_ISSUER\" --api-secret \"$AMO_JWT_SECRET\""
   },
   "devDependencies": {
-    "pdf-lib": "^1.17.1"
+    "pdf-lib": "^1.17.1",
+    "web-ext": "^8.3.0"
   }
 }