Add Rails backend: accounts via magic link, server documents, sharing

The app becomes a Rails 8 application; the Vue frontend moves to
app/frontend and is served through vite_rails.

- Without an account documents stay in the browser (IndexedDB). Sign-in
  is by magic link or the 6-digit code from the same mail; afterwards
  local documents can be uploaded into the account.
- Server documents (/d/<id>) with versioned saves and a single-editor
  lock per browser tab; others see them read-only and follow by polling.
- Sharing via link (works without an account) and via e-mail invitation,
  roles viewer/editor; the owner manages links and people.
- Store gains a read-only mode that blocks all content changes.
- Solid Queue, Cache and Cable share the one PostgreSQL database.

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

app/frontend/io/operations.ts

@@ -0,0 +1,131 @@
+/**
+ * High-level import/export operations. Each one is atomic: on any failure it
+ * throws and leaves the document untouched (the UI catches and shows a toast).
+ */
+
+import type { DocumentStore } from '../store/document';
+import { downloadBlob, downloadText } from './download';
+import { serializeDocument, parseDocument, reidSheets } from './json';
+import {
+  parseCsv,
+  sniffDelimiter,
+  matrixToCells,
+  tableToCsv,
+  type Delimiter,
+  type CsvExportOptions,
+} from './csv';
+import { documentToXlsx, xlsxToSheets, importedToSheet } from './xlsx';
+
+export function readAsText(file: File): Promise<string> {
+  return new Promise((resolve, reject) => {
+    const fr = new FileReader();
+    fr.onerror = () => reject(new Error('Datei konnte nicht gelesen werden.'));
+    fr.onload = () => resolve(String(fr.result));
+    fr.readAsText(file);
+  });
+}
+
+export function readAsArrayBuffer(file: File): Promise<ArrayBuffer> {
+  return new Promise((resolve, reject) => {
+    const fr = new FileReader();
+    fr.onerror = () => reject(new Error('Datei konnte nicht gelesen werden.'));
+    fr.onload = () => resolve(fr.result as ArrayBuffer);
+    fr.readAsArrayBuffer(file);
+  });
+}
+
+export function extensionOf(name: string): string {
+  const i = name.lastIndexOf('.');
+  return i >= 0 ? name.slice(i + 1).toLowerCase() : '';
+}
+
+/** Yield to the event loop so a large parse doesn't freeze the UI. */
+const breathe = () => new Promise((r) => setTimeout(r, 0));
+
+// ---- export ---------------------------------------------------------
+
+/** A document title as a file name (without extension). */
+export function fileBaseName(title: string): string {
+  return title.replace(/[\\/:*?"<>|\u0000-\u001f]+/g, '-').trim() || 'dokument';
+}
+
+export function exportJson(store: DocumentStore, name = 'dokument'): void {
+  downloadText(`${fileBaseName(name)}.json`, serializeDocument(store.snapshot()), 'application/json');
+}
+
+/** Save as the native `.num` file (the JSON document, zipped -- see num.ts).
+ *  The zip code loads on first use, like the xlsx support. */
+export async function exportNum(store: DocumentStore, name = 'dokument'): Promise<void> {
+  const { packNum, NUM_MIMETYPE } = await import('./num');
+  const bytes = packNum(serializeDocument(store.snapshot()));
+  downloadBlob(`${fileBaseName(name)}.num`, new Blob([bytes], { type: NUM_MIMETYPE }));
+}
+
+/** The document JSON inside a `.num` file -- then imported like a `.json`. */
+export async function readNumFile(buffer: ArrayBuffer): Promise<string> {
+  const { unpackNum } = await import('./num');
+  return unpackNum(new Uint8Array(buffer));
+}
+
+export function exportActiveTableCsv(store: DocumentStore, opts: CsvExportOptions): void {
+  const table = store.activeTable.value;
+  if (!table) throw new Error('Keine Tabelle ausgewählt.');
+  downloadText(`${table.name}.csv`, tableToCsv(store, table.id, opts), 'text/csv;charset=utf-8');
+}
+
+export async function exportXlsx(store: DocumentStore, name = 'arbeitsmappe'): Promise<void> {
+  downloadBlob(`${fileBaseName(name)}.xlsx`, await documentToXlsx(store));
+}
+
+// ---- import: JSON ------------------------------------------------
+
+export function importJsonReplace(store: DocumentStore, text: string): void {
+  store.replaceDocument(parseDocument(text)); // throws JsonImportError on bad schema
+}
+
+export function importJsonAppend(store: DocumentStore, text: string): void {
+  store.appendSheets(reidSheets(parseDocument(text)));
+}
+
+// ---- import: CSV -----------------------------------------------
+
+export interface CsvImportConfig {
+  mode: 'new-table' | 'replace-active';
+  delimiter: Delimiter | 'auto';
+  detectNumbers: boolean;
+  name?: string;
+}
+
+export async function importCsv(
+  store: DocumentStore,
+  text: string,
+  cfg: CsvImportConfig,
+): Promise<void> {
+  const delimiter: Delimiter = cfg.delimiter === 'auto' ? sniffDelimiter(text) : cfg.delimiter;
+  const matrix = parseCsv(text, delimiter);
+  if (matrix.length > 2000) await breathe();
+  const { cells, rows, cols } = matrixToCells(matrix, cfg.detectNumbers);
+
+  if (cfg.mode === 'replace-active') {
+    const table = store.activeTable.value;
+    if (!table) throw new Error('Keine Zieltabelle ausgewählt.');
+    store.replaceTableCells(table.id, cells, Math.max(rows, 12), Math.max(cols, 6));
+  } else {
+    store.addTableFromCells(
+      store.doc.activeSheetId,
+      cfg.name?.trim() || 'Import',
+      cells,
+      Math.max(rows, 12),
+      Math.max(cols, 6),
+    );
+  }
+}
+
+// ---- import: XLSX --------------------------------------------
+
+export async function importXlsxFile(store: DocumentStore, buffer: ArrayBuffer): Promise<void> {
+  const imported = await xlsxToSheets(buffer);
+  if (imported.length === 0) throw new Error('Die Arbeitsmappe enthält keine Tabellen.');
+  await breathe();
+  store.appendSheets(imported.map((s, i) => importedToSheet(s, i)));
+}