ui: tidy job-page download into a single dropdown

Replace the floating 'Download log' button + bare '.ndjson' link with
one cohesive dropdown menu — same affordance as the rest of the
header, opens to two well-described options.

- Native <details><summary> for keyboard + no-JS support; only the
  click-outside-to-close handler is JS (a few lines).
- New .dropdown / .dropdown-menu / .dropdown-item tokens in
  web/styles/input.css. Reusable for future header menus
  (host-detail overflow, source-group action menus, etc).
- Chevron flips 180 degrees when open via .dropdown[open] selector.
- Each option has a label + a mono hint line explaining when to pick it
  (.txt for humans / paste into a ticket; .ndjson for jq / tooling).

web/templates/pages/job_detail.html

@@ -63,11 +63,22 @@
       </div>
     </div>
     <div class="flex items-center gap-2">
-      <a href="/api/jobs/{{$job.ID}}/log.txt" class="btn"
-         title="Download the full log up to this moment (works while running too).">
-        Download log
-      </a>
-      <a href="/api/jobs/{{$job.ID}}/log.ndjson" class="btn btn-ghost" title="Same log as NDJSON for jq / tooling">.ndjson</a>
+      <details class="dropdown" id="download-menu">
+        <summary>
+          Download log
+          <span class="chev">▾</span>
+        </summary>
+        <div class="dropdown-menu">
+          <a class="dropdown-item" href="/api/jobs/{{$job.ID}}/log.txt">
+            <span class="label">Plain text</span>
+            <span class="hint">.txt · for humans / paste into a ticket</span>
+          </a>
+          <a class="dropdown-item" href="/api/jobs/{{$job.ID}}/log.ndjson">
+            <span class="label">JSON Lines</span>
+            <span class="hint">.ndjson · pipe into jq / tooling</span>
+          </a>
+        </div>
+      </details>
       {{if $page.IsActive}}
         <button class="btn btn-danger" id="cancel-btn"
                 hx-post="/api/jobs/{{$job.ID}}/cancel"
@@ -76,6 +87,16 @@
         <a href="/hosts/{{$host.ID}}" class="btn">Back to host</a>
       {{end}}
     </div>
+    <script>
+    // Close the download dropdown when clicking outside it.
+    (function() {
+      var dd = document.getElementById('download-menu');
+      if (!dd) return;
+      document.addEventListener('click', function(e) {
+        if (dd.open && !dd.contains(e.target)) dd.open = false;
+      });
+    })();
+    </script>
   </div>
 
   {{/* ---------- progress (running only) ---------- */}}