Combat tracker: multiple named encounters

- Encounter entity; Combatant now belongs to an encounter (ON DELETE
  CASCADE). Migration adds encounter_id, rehoming any existing combatants.
- /tracker lists encounters (name, count, last updated) with a "New
  encounter" form; /tracker/{id} is the HP tracker for one encounter.
- Encounters can be renamed, have their combatants cleared, or be deleted.
- The Enemies list gains an encounter picker next to "Add to tracker"
  (existing encounter or "— New encounter —", which prompts for a name);
  an encounter's "Add enemies" link pre-selects it via ?encounter=.
- Routes for combatant HP/remove moved under /tracker/combatant/ to keep
  the numeric {id} for encounters.

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

assets/controllers/tracker_picker_controller.js

@@ -25,11 +25,21 @@ export default class extends Controller {
             return;
         }
 
+        const select = this.element.querySelector('.encounter-select');
+        let encounter = select ? select.value : 'new';
+        let name = '';
+        if (encounter === 'new') {
+            name = window.prompt('Name for the new encounter', 'Encounter');
+            if (name === null) return; // cancelled
+        }
+
         const form = document.createElement('form');
         form.method = 'post';
         form.action = this.urlValue;
         form.insertAdjacentHTML('beforeend',
-            `<input type="hidden" name="_token" value="${this.tokenValue}">`);
+            `<input type="hidden" name="_token" value="${this.tokenValue}">` +
+            `<input type="hidden" name="encounter" value="${encounter}">` +
+            `<input type="hidden" name="name" value="${name.replace(/"/g, '&quot;')}">`);
         picks.forEach((p, i) => {
             form.insertAdjacentHTML('beforeend',
                 `<input type="hidden" name="picks[${i}][id]" value="${p.id}">` +