feat(sacred-valley): drag-to-reorder with server-persisted layout

Adds HTML5 drag-to-reorder for .sv-card elements in Sacred Valley. The
pure moveId helper is unit-tested. Drop calls PUT /api/dashboard/layout
to persist the new card_order; DOM reflects the new order immediately.

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

public/views/sacred_valley.js

@@ -1,6 +1,7 @@
 import { el, mount } from '../dom.js';
 import { api } from '../api.js';
 import { svCard } from '../components/sv_card.js';
+import { attachReorder } from '../components/sv_reorder.js';
 import { orderCards } from './cards/registry.js';
 import clock from './cards/clock.js';
 import weather from './cards/weather.js';
@@ -30,5 +31,13 @@ export async function render(main) {
     try { def.mount(body); def.start && def.start(); active.push(def); }
     catch (e) { body.appendChild(el('span', { class: 'muted' }, 'card failed')); console.error(def.id, e); }
   }
-  // health band + drag wiring arrive in Tasks 22 and 10.
+  attachReorder(grid, async (newOrder) => {
+    // reflect immediately
+    const frag = document.createDocumentFragment();
+    newOrder.forEach(id => { const n = grid.querySelector(`.sv-card[data-card-id="${id}"]`); if (n) frag.appendChild(n); });
+    grid.appendChild(frag);
+    try { await api.put('/api/dashboard/layout', { ...layout, card_order: newOrder }); layout.card_order = newOrder; }
+    catch (e) { console.error('save layout', e); }
+  });
+  // health band wiring arrives in Task 22.
 }