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>

assets/controllers/tracker_picker_controller.js

@@ -0,0 +1,41 @@
+import { Controller } from '@hotwired/stimulus';
+
+/*
+ * Collects the ticked rows (+ their quantity) from a DataTable and POSTs them
+ * to the combat tracker.
+ *
+ *   <div data-controller="tracker-picker"
+ *        data-tracker-picker-url-value="/tracker/add"
+ *        data-tracker-picker-token-value="...">
+ *     <button data-action="tracker-picker#submit">Add to tracker</button>
+ *   </div>
+ */
+export default class extends Controller {
+    static values = { url: String, token: String };
+
+    submit() {
+        const picks = [...document.querySelectorAll('.track-check:checked')].map((cb) => {
+            const qtyEl = cb.closest('.track-pick')?.querySelector('.track-qty');
+            const qty = Math.max(1, Math.min(50, parseInt(qtyEl?.value ?? '1', 10) || 1));
+            return { id: cb.value, qty };
+        });
+
+        if (picks.length === 0) {
+            window.alert('Tick at least one enemy first.');
+            return;
+        }
+
+        const form = document.createElement('form');
+        form.method = 'post';
+        form.action = this.urlValue;
+        form.insertAdjacentHTML('beforeend',
+            `<input type="hidden" name="_token" value="${this.tokenValue}">`);
+        picks.forEach((p, i) => {
+            form.insertAdjacentHTML('beforeend',
+                `<input type="hidden" name="picks[${i}][id]" value="${p.id}">` +
+                `<input type="hidden" name="picks[${i}][qty]" value="${p.qty}">`);
+        });
+        document.body.appendChild(form);
+        form.requestSubmit();
+    }
+}