Add core domain layer: schema, store, sanitizer, placeholders, settings

- Folder-based GuideStore with atomic writes, trash/restore, duplicate,
  substep reparenting, folders/favorites, working-image crop/reset
- Allowlist HTML sanitizer applied on every store write
- Placeholder scopes (guide > global > system) and collection
- Persisted app settings with deep default merge
- 16 workflow tests exercising real on-disk round-trips

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

core/placeholders.js

@@ -0,0 +1,79 @@
+'use strict';
+
+const { htmlToText } = require('./util');
+
+/**
+ * Placeholders are [[Name]] tokens usable in titles, descriptions, text
+ * blocks, and export cover pages. Resolution precedence (highest wins):
+ * guide placeholders > global placeholders > system placeholders.
+ * Unknown tokens are left untouched so typos are visible in output.
+ */
+
+const TOKEN_RE = /\[\[([A-Za-z0-9_ .-]+)\]\]/g;
+
+function systemPlaceholders(guide, { now = new Date(), stepCount = null } = {}) {
+  const pad = (n) => String(n).padStart(2, '0');
+  const date = `${now.getFullYear()}-${pad(now.getMonth() + 1)}-${pad(now.getDate())}`;
+  const time = `${pad(now.getHours())}:${pad(now.getMinutes())}`;
+  return {
+    Guide_Title: guide ? guide.title : '',
+    Guide_Description: guide ? htmlToText(guide.descriptionHtml) : '',
+    Date: date,
+    Time: time,
+    DateTime: `${date} ${time}`,
+    Year: String(now.getFullYear()),
+    Step_Count: stepCount == null ? '' : String(stepCount),
+    App_Name: 'StepForge',
+  };
+}
+
+/** Build the effective name->value map for a guide. */
+function resolveScopes({ guide = null, globals = {}, system = {} } = {}) {
+  return { ...system, ...globals, ...(guide && guide.placeholders ? guide.placeholders : {}) };
+}
+
+function expandPlaceholders(text, values) {
+  if (!text) return text == null ? '' : text;
+  return String(text).replace(TOKEN_RE, (whole, name) => {
+    const key = name.trim();
+    return Object.prototype.hasOwnProperty.call(values, key) && values[key] != null
+      ? String(values[key])
+      : whole;
+  });
+}
+
+/** List distinct placeholder names used in a string. */
+function listPlaceholders(text) {
+  const names = new Set();
+  if (text) {
+    for (const m of String(text).matchAll(TOKEN_RE)) names.add(m[1].trim());
+  }
+  return [...names];
+}
+
+/** Collect every placeholder name used anywhere in a guide + its steps. */
+function collectGuidePlaceholders(guide, steps) {
+  const names = new Set();
+  const add = (text) => listPlaceholders(text).forEach((n) => names.add(n));
+  add(guide.title);
+  add(guide.descriptionHtml);
+  for (const step of steps) {
+    add(step.title);
+    add(step.descriptionHtml);
+    for (const tb of step.textBlocks || []) {
+      add(tb.title);
+      add(tb.descriptionHtml);
+    }
+    for (const ann of step.annotations || []) add(ann.text);
+  }
+  return [...names].sort();
+}
+
+module.exports = {
+  TOKEN_RE,
+  systemPlaceholders,
+  resolveScopes,
+  expandPlaceholders,
+  listPlaceholders,
+  collectGuidePlaceholders,
+};