feat(theme-registry): buildThemeMenu converts registry → Electron MenuItem[]

Adds src/main/themeMenuBuilder.js — a pure module that consumes
ThemeRegistry.list() + ThemeRegistry.categories() and the injected
setTheme/getCurrentThemeId callbacks to produce the View → Theme
submenu's MenuItemTemplate[] in the same shape as the previous
hardcoded block in src/main.js:1137-1245.

- Radio-style items with checked=true on the active theme id
- Grouped by category in registry order with separators between
  non-empty categories
- No Electron / electron-store imports — keeps the module pure and
  unit-testable under @jest-environment node
- Tests use jest.resetModules() + per-test require to avoid the
  module-cache leakage the bootstrap test surfaced in T2

Amit Haridas

src/main/themeMenuBuilder.js

@@ -0,0 +1,50 @@
+/**
+ * Editor Theme menu builder — converts ThemeRegistry output into Electron
+ * MenuItem[] suitable for the View → Theme submenu.
+ *
+ * Pure: takes `setTheme` + `getCurrentThemeId` as injected dependencies so
+ * the builder never touches electron-store or `mainWindow`. Tested in
+ * isolation under `@jest-environment node`.
+ *
+ * Menu shape (matches the previous hardcoded block in src/main.js:1137-1245):
+ *   [item, item, …, { type: 'separator' }, item, item, …, separator, …]
+ * grouped by category in registry order, with separators between non-empty
+ * categories. Each theme item is a radio-style MenuItem so Electron shows a
+ * checkmark next to the active theme.
+ *
+ * @module themeMenuBuilder
+ */
+
+const ThemeRegistry = require('./ThemeRegistry');
+
+/**
+ * @param {object} deps
+ * @param {(id: string) => void} deps.setTheme
+ * @param {() => string} deps.getCurrentThemeId
+ * @returns {Array<object>} Electron MenuItemTemplate[]
+ */
+function buildThemeMenu({ setTheme, getCurrentThemeId }) {
+  const currentId = getCurrentThemeId();
+  const items = [];
+  const cats = ThemeRegistry.categories();
+  let nonEmptySeen = 0;
+
+  for (const cat of cats) {
+    const inCat = ThemeRegistry.list().filter((t) => t.category === cat);
+    if (inCat.length === 0) continue;
+    if (nonEmptySeen > 0) items.push({ type: 'separator' });
+    nonEmptySeen++;
+
+    for (const t of inCat) {
+      items.push({
+        label: t.label,
+        type: 'radio',
+        checked: t.id === currentId,
+        click: () => setTheme(t.id),
+      });
+    }
+  }
+  return items;
+}
+
+module.exports = { buildThemeMenu };