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/io/json.ts

@@ -11,6 +11,7 @@
 import type {
   CellFormat,
   ChartObject,
+  CustomFunction,
   DocumentState,
   HAlign,
   Sheet,
@@ -32,6 +33,30 @@ function localeOrUndef(x: unknown): NumberLocale | undefined {
   return isNumberLocale(x) ? x : undefined;
 }
 
+/** Defaults to `[]` -- absent in every document saved before this feature
+ *  existed (and in v1 documents entirely) -- and drops any malformed entry
+ *  rather than rejecting the whole import over one bad function. */
+function normaliseCustomFunctions(x: unknown): CustomFunction[] {
+  if (!Array.isArray(x)) return [];
+  const out: CustomFunction[] = [];
+  for (const f of x) {
+    if (
+      !f ||
+      typeof f !== 'object' ||
+      typeof (f as { id?: unknown }).id !== 'string' ||
+      typeof (f as { name?: unknown }).name !== 'string' ||
+      typeof (f as { body?: unknown }).body !== 'string' ||
+      !Array.isArray((f as { params?: unknown }).params) ||
+      !(f as { params: unknown[] }).params.every((p) => typeof p === 'string')
+    ) {
+      continue;
+    }
+    const rec = f as CustomFunction;
+    out.push({ id: rec.id, name: rec.name, params: [...rec.params], body: rec.body });
+  }
+  return out;
+}
+
 const HEX_COLOR = /^#(?:[0-9a-f]{3}|[0-9a-f]{4}|[0-9a-f]{6}|[0-9a-f]{8})$/i;
 function colorOrUndef(x: unknown): string | undefined {
   return typeof x === 'string' && HEX_COLOR.test(x) ? x : undefined;
@@ -303,6 +328,7 @@ export function validateDocument(x: unknown): DocumentState {
         : null,
     locale: isNumberLocale(doc.locale) ? doc.locale : DEFAULT_LOCALE,
     csvDelimiter: doc.csvDelimiter === ';' ? ';' : ',',
+    customFunctions: normaliseCustomFunctions(doc.customFunctions),
   };
 }
 
@@ -365,6 +391,7 @@ export function migrateToV2(x: unknown): DocumentState {
         : null,
     locale: DEFAULT_LOCALE,
     csvDelimiter: ',',
+    customFunctions: [],
   };
 }