Add user-defined functions, editable exclusively via a new UI

Document-wide, named formula functions (e.g. MARGE(revenue; cost) =
(revenue - cost) / revenue) callable from any cell like a built-in
(=MARGE(Umsatz!B2; Umsatz!C2)). Created, edited and deleted only through
a new "𝑓 Funktionen" toolbar button -> modal (list + shared edit form),
never via JSON handedit.

Engine: a call is inlined into the caller's AST at parse time
(src/engine/userFunctions.ts's expandUserFunctions), so evaluator.ts and
graph.ts need no changes -- dependencies on cells referenced only inside
a function body (not in the call) are picked up for free. parser.ts
gains an optional paramNames argument to parseFormula() producing a new
ParamNode leaf, used only when parsing a function body.

Store: addCustomFunction/updateCustomFunction/deleteCustomFunction with
inline (not toast) validation -- name/parameter shape, no collision with
a built-in or another function, the body parses, and a definition-time
cycle check across the whole call graph. Renaming/deleting a function
mirrors table rename/delete: existing calls are not rewritten, they
become #NAME? on the next rebuild. Fully undo/redo-able and part of the
JSON document (customFunctions: [], migrated for v1 and older v2 docs).

178 tests passing (32 new), vue-tsc and eslint clean. Manually verified
end-to-end in the running app via Playwright: define/call/recalculate,
rename -> #NAME?, a closing cycle rejected inline at save, undo restores
the function library.

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

src/engine/index.ts

@@ -31,6 +31,12 @@ export {
   literalValue,
   type TableModel,
 } from './workbook';
+export {
+  buildFunctionIndex,
+  expandUserFunctions,
+  collectCalledFunctionNames,
+  type FunctionDef,
+} from './userFunctions';
 export {
   FormulaError,
   isError,