feat(app): editor engine, markdown, store and domain modules

Block editor, markdown round trip, formulas/databases/kanban/canvas
models, graph simulation (time-budgeted ticks for big vaults), shortcut
registry, dialog focus handling, HTML to Markdown for paste and clipping.

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

src/lib/api.js

@@ -0,0 +1,12 @@
+// In Electron, window.vault comes from electron/preload.cjs.
+// In a plain browser (`npm run dev:web`) an in-memory mock stands in, so the UI can be worked on without Electron.
+import { createMock } from './mock.js';
+
+export const isDesktop = typeof window !== 'undefined' && !!window.vault;
+export const api = isDesktop ? window.vault : createMock();
+export const platform = isDesktop ? window.vault.platform : 'web';
+
+export const errorText = (err) =>
+  String((err && err.message) || err || 'Something went wrong')
+    .replace(/^Error invoking remote method '[^']+': /, '')
+    .replace(/^Error: /, '');