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/api.js

@@ -63,6 +63,7 @@ function promptForToken() {
 export const api = {
   get:   (p)        => call('GET',    p),
   post:  (p, body)  => call('POST',   p, body ?? {}),
+  put:   (p, body)  => call('PUT',    p, body ?? {}),
   patch: (p, body)  => call('PATCH',  p, body ?? {}),
   del:   (p)        => call('DELETE', p),
   setToken: (v) => localStorage.setItem(TOKEN_KEY, v),

public/components/sv_reorder.js

@@ -0,0 +1,28 @@
+export function moveId(order, dragId, beforeId) {
+  if (dragId === beforeId) return [...order];
+  const out = order.filter(id => id !== dragId);
+  if (beforeId == null) { out.push(dragId); return out; }
+  const i = out.indexOf(beforeId);
+  if (i < 0) { out.push(dragId); return out; }
+  out.splice(i, 0, dragId);
+  return out;
+}
+
+// Wires HTML5 drag on .sv-card elements in `grid`. onReorder(newOrderIds) fires
+// after a drop. Drag handle = the whole card (cursor:grab on the title via CSS).
+export function attachReorder(grid, onReorder) {
+  let dragId = null;
+  grid.querySelectorAll('.sv-card').forEach(card => {
+    card.draggable = true;
+    card.addEventListener('dragstart', () => { dragId = card.dataset.cardId; card.classList.add('dragging'); });
+    card.addEventListener('dragend', () => { card.classList.remove('dragging'); dragId = null; });
+    card.addEventListener('dragover', e => { e.preventDefault(); card.classList.add('drag-over'); });
+    card.addEventListener('dragleave', () => card.classList.remove('drag-over'));
+    card.addEventListener('drop', e => {
+      e.preventDefault(); card.classList.remove('drag-over');
+      if (!dragId || dragId === card.dataset.cardId) return;
+      const ids = [...grid.querySelectorAll('.sv-card')].map(c => c.dataset.cardId);
+      onReorder(moveId(ids, dragId, card.dataset.cardId));
+    });
+  });
+}

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.
 }

tests/frontend/reorder.test.js

@@ -0,0 +1,14 @@
+import { describe, it, expect } from 'vitest';
+import { moveId } from '../../public/components/sv_reorder.js';
+
+describe('moveId', () => {
+  it('moves an id before a target', () => {
+    expect(moveId(['a', 'b', 'c'], 'c', 'a')).toEqual(['c', 'a', 'b']);
+  });
+  it('moving onto itself is a no-op', () => {
+    expect(moveId(['a', 'b', 'c'], 'b', 'b')).toEqual(['a', 'b', 'c']);
+  });
+  it('moving to end when target is null appends', () => {
+    expect(moveId(['a', 'b', 'c'], 'a', null)).toEqual(['b', 'c', 'a']);
+  });
+});