Add combat HP tracker

- one running tracker (Combatant rows); no encounters to manage
- Enemies list gets a "Track" column (checkbox + count) and an
  "Add to tracker" button; picking N of an enemy makes "Goblin 1..N",
  numbering continues if some are already on the tracker
- /tracker: per-combatant current/max HP with an HP bar, Damage / Heal
  by amount, remove one, "Clear all". HP is clamped to [0, max];
  0 HP shows a "Down" badge. Stats are snapshotted from the Enemy on add.
- "Tracker" nav item; tracker_picker Stimulus controller; app.js gains a
  data-format="pick" cell renderer

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/enemy/index.html.twig

@@ -3,9 +3,17 @@
 {% block title %}Enemies · dmtools{% endblock %}
 
 {% block body %}
-    <div class="d-flex justify-content-between align-items-center mb-3">
+    <div class="d-flex justify-content-between align-items-center mb-3 gap-2 flex-wrap">
         <h1 class="h3 mb-0">Enemies</h1>
-        <a href="{{ path('app_enemies_new') }}" class="btn btn-primary btn-sm">Add enemy</a>
+        <div class="d-flex gap-2"
+             data-controller="tracker-picker"
+             data-tracker-picker-url-value="{{ path('app_tracker_add') }}"
+             data-tracker-picker-token-value="{{ csrf_token('track-add') }}">
+            <button type="button" class="btn btn-outline-primary btn-sm" data-action="tracker-picker#submit">
+                Add to tracker
+            </button>
+            <a href="{{ path('app_enemies_new') }}" class="btn btn-primary btn-sm">Add enemy</a>
+        </div>
     </div>
 
     <div class="card shadow-sm">
@@ -22,6 +30,7 @@
                         <th data-data="challengeRating">CR</th>
                         <th data-data="armorClass">AC</th>
                         <th data-data="gameNotes" data-format="truncate" data-orderable="false">Game notes</th>
+                        <th data-data="id" data-format="pick" data-orderable="false">Track</th>
                     </tr>
                 </thead>
                 <tbody></tbody>