Add cancel job action

POST /jobs/{id}/cancel sets status → cancelled, frees printer if printing,
logs 'Job cancelled'. Button visible in all non-terminal states (queued, printing).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/templates/job_detail.html

@@ -27,14 +27,13 @@
 
 {# ── Job actions ── #}
 {% set s = job.status.value %}
-{% if s == 'queued' %}
-<div class="mb-4">
+{% if s not in ('done', 'cancelled') %}
+<div class="d-flex gap-2 mb-4">
+  {% if s == 'queued' %}
   <button class="btn btn-success btn-sm" data-bs-toggle="modal" data-bs-target="#startModal">
     <i class="bi bi-play-fill me-1"></i>Start Job
   </button>
-</div>
-{% elif s == 'printing' %}
-<div class="d-flex gap-2 mb-4">
+  {% elif s == 'printing' %}
   <form method="POST" action="/jobs/{{ job.id }}/finish">
     <button type="submit" class="btn btn-success btn-sm">
       <i class="bi bi-check-lg me-1"></i>Finish
@@ -45,6 +44,12 @@
       <i class="bi bi-stop-fill me-1"></i>Stop
     </button>
   </form>
+  {% endif %}
+  <form method="POST" action="/jobs/{{ job.id }}/cancel">
+    <button type="submit" class="btn btn-outline-danger btn-sm">
+      <i class="bi bi-x-lg me-1"></i>Cancel Job
+    </button>
+  </form>
 </div>
 {% endif %}