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/engine/index.ts

@@ -0,0 +1,81 @@
+/**
+ * Public surface of the formula engine.
+ *
+ * Nothing in `./` imports Vue, the DOM or any spreadsheet library. See
+ * ./README.md for the grammar (EBNF), the evaluation model and the error model.
+ */
+
+export { tokenize, LexError, type Token, type TokenType } from './lexer';
+export {
+  parseFormula,
+  parseFunctionBlock,
+  parseMacroBlock,
+  collectRefs,
+  collectRefsInBlock,
+  ParseError,
+  type Node,
+  type CellNode,
+  type RangeNode,
+  type NamedRefNode,
+  type BlockNode,
+  type ActionNode,
+} from './parser';
+export { evaluate, runMacroActions } from './evaluator';
+export { DependencyGraph } from './graph';
+export { FUNCTIONS, getFunction, hasFunction, BUILTIN_SIGNATURES } from './functions';
+export {
+  colToNumber,
+  numberToCol,
+  parseA1,
+  toA1,
+  looksLikeCell,
+  expandRange,
+  rangeCells,
+  wholeLineAt,
+  parseLinePart,
+  formatLinePart,
+  WHOLE_LINE_END,
+  cycleReferenceLocks,
+  currentIdentifierAtCaret,
+} from './refs';
+export {
+  Workbook,
+  literalValue,
+  type TableModel,
+} from './workbook';
+export {
+  dateToSerial,
+  timeSerial,
+  serialToUtcDate,
+  todaySerial,
+  nowSerial,
+  parseDateTimeLiteral,
+  type DateLiteralKind,
+} from './dates';
+export {
+  buildFunctionIndex,
+  expandUserFunctions,
+  expandMacroBlock,
+  collectCalledFunctionNames,
+  collectCalledFunctionNamesInBlock,
+  declaredBlockNames,
+  findInvalidForSource,
+  findInvalidActionTarget,
+  blockAlwaysReturns,
+  type FunctionDef,
+} from './userFunctions';
+export {
+  FormulaError,
+  isError,
+  keyOf,
+  parseKey,
+  ERROR_CODES,
+  type ErrorCode,
+  type CellValue,
+  type CellRef,
+  type Scalar,
+  type EngineHost,
+  type MacroWriteHost,
+  type MacroNetworkHost,
+  type NamedRangeDef,
+} from './types';