feat(sacred-valley): hybrid free/snap canvas + blank & blackflame cards (2.8.0)

Replace masonry grid with an absolute-positioned 12-col canvas: drag to
move, corner to resize, per-card free/overlap toggle (Alt = no-snap).
Geometry persisted (migration 027: dashboard_layout.geom + extras).
Two new addable decorative cards: blank spacer + animated blackflame
(canvas particle flame). Old layout auto-migrates by flow-placement.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/views/cards/blank.js

@@ -0,0 +1,16 @@
+// A decorative blank spacer card — deliberate empty space / grouping on the
+// Sacred Valley canvas. Instanceable: each gets a unique id via the factory.
+import { el } from '../../dom.js';
+
+export function blankCard(id) {
+  return {
+    id,
+    type: 'blank',
+    title: '',
+    decorative: true,
+    size: 'm',
+    mount(body) { body.appendChild(el('div', { class: 'sv-blank' })); },
+    start() {},
+    stop() {}
+  };
+}