feat(site-builder): block registry, notices, local prefs, project extras, overlay slot

Add an exported block registry (keywords, search, insertBlock) consumed by
BlocksPanel, a NoticeProvider/NoticeHost toast stack, a guarded localStorage
prefs helper, a ProjectExtras store persisted as project.json 'extras', and a
CanvasOverlays mount point.

Refs #38
Refs #41
Refs #49
Refs #48
Refs #57
Refs #58

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

craft/src/styles/editor.css

@@ -2054,3 +2054,45 @@ body {
   padding: 4px 12px;
 }
 .conflict-modal-actions button:disabled { opacity: 0.5; cursor: default; }
+
+/* Notice stack (state/NoticeContext + editor/NoticeHost): bottom-left, so it
+   never collides with TopBar's bottom-right ErrorNotice. */
+.notice-host {
+  position: fixed;
+  left: 16px;
+  bottom: 16px;
+  z-index: 60;
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+  max-width: min(420px, calc(100vw - 32px));
+}
+.notice-item {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 10px 12px;
+  font-size: 12px;
+  color: var(--color-text);
+  background: var(--color-bg-elevated);
+  border: 1px solid var(--color-border);
+  border-left: 3px solid var(--color-border);
+  border-radius: var(--radius-md);
+  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.4);
+}
+.notice-item.success { border-left-color: var(--color-success); }
+.notice-item.error { border-left-color: var(--color-danger); }
+.notice-text { flex: 1; word-break: break-word; }
+.notice-action, .notice-dismiss {
+  background: none;
+  border: none;
+  color: inherit;
+  cursor: pointer;
+  padding: 0 4px;
+}
+.notice-action { font-weight: 600; text-decoration: underline; }
+@media (max-width: 768px) {
+  .notice-host {
+    bottom: calc(var(--mobile-tabbar-height) + env(safe-area-inset-bottom, 0px) + 12px);
+  }
+}