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.

popup.html

@@ -36,10 +36,18 @@
     <h2>PDF Settings</h2>
     <label class="row"><input type="checkbox" id="truncateToggle"> Truncate long course descriptions (official PDF style)</label>
     <label class="row"><input type="checkbox" id="showYearToggle"> Show Year Level on the PDF</label>
+    <div class="row">Year Level mode:
+      <select id="yearMode">
+        <option value="auto">Auto (recommended)</option>
+        <option value="custom">Use custom value</option>
+        <option value="off">Profile value only</option>
+      </select>
+    </div>
     <label class="row">Custom Year Level (optional):
       <input type="text" id="customYear" placeholder="e.g. 3">
     </label>
-    <p class="hint">Leave blank to use the scraped value (auto-adjusted in Summer).</p>
+    <p class="hint">Auto derives the standing of the semester being printed (from
+      View Grades, or remaining units). Profile value only disables derivation.</p>
   </section>
 
   <script src="popup.js"></script>