Show a range-handling pattern in the function-editing UI

A user function parameter already worked as a range when passed
straight into a range-capable call (SUM/AVG/MIN/MAX/another user
function) -- expandUserFunctions splices the caller's RangeNode in
unchanged, so it behaves exactly like a normal formula (and #VALUE!
outside a function-argument position, also exactly like normal). Added
2 tests confirming this (incl. recalculation on a change inside the
passed range) and a hint under the Formel field in
CustomFunctionsModal.vue showing the pattern (SPANNE(bereich) =
MAX(bereich) - MIN(bereich)), since nothing surfaced that this was
possible.

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

src/components/CustomFunctionsModal.vue

@@ -146,6 +146,13 @@ function close() {
           placeholder="z. B. (revenue - cost) / revenue"
         />
       </label>
+      <p class="hint">
+        Ein Parameter kann auch ein <strong>Bereich</strong> sein, solange er
+        direkt an eine Funktion wie <code>SUM</code>/<code>AVG</code>/
+        <code>MIN</code>/<code>MAX</code> übergeben wird — z. B.
+        <code>SPANNE(bereich) = MAX(bereich) - MIN(bereich)</code>, aufgerufen
+        als <code>=SPANNE(Umsatz!B2:E6)</code>.
+      </p>
 
       <p v-if="formError" class="form-error">⚠ {{ formError }}</p>
     </template>

src/engine/__tests__/userFunctions.spec.ts

@@ -199,6 +199,34 @@ describe('Workbook integration: user-defined functions end-to-end', () => {
     expect((val(wb, 's-umsatz', 'B9') as FormulaError).code).toBe('#DIV/0!');
   });
 
+  it('a parameter can be a range, as long as it lands in a function-call argument position', () => {
+    // SPANNE(bereich) = MAX(bereich) - MIN(bereich) -- "bereich" (ParamNode)
+    // gets substituted with the caller's RangeNode, splicing a normal
+    // MAX(Umsatz!B2:E2) / MIN(Umsatz!B2:E2) call into the expanded AST.
+    const umsatz = sheet('s-umsatz', 'Umsatz', {
+      B2: '1200', C2: '1450', D2: '1600', E2: '1720',
+      B9: '=SPANNE(B2:E2)',
+    });
+    const wb = new Workbook(
+      [umsatz],
+      [def({ id: 'f1', name: 'SPANNE', params: ['bereich'], body: 'MAX(bereich) - MIN(bereich)' })],
+    );
+    expect(val(wb, 's-umsatz', 'B9')).toBe(1720 - 1200);
+
+    // recalculates when a cell *inside* the range argument changes
+    wb.setCell('s-umsatz', 1, 1, '1000'); // B2: 1200 -> 1000
+    expect(val(wb, 's-umsatz', 'B9')).toBe(1720 - 1000);
+  });
+
+  it('a range parameter used outside a function-call argument position is #VALUE!, same as a bare range in an ordinary formula', () => {
+    const umsatz = sheet('s-umsatz', 'Umsatz', {
+      B2: '1200', C2: '1450',
+      B9: '=BAD(B2:C2)',
+    });
+    const wb = new Workbook([umsatz], [def({ id: 'f1', name: 'BAD', params: ['bereich'], body: 'bereich + 1' })]);
+    expect((val(wb, 's-umsatz', 'B9') as FormulaError).code).toBe('#VALUE!');
+  });
+
   it('debugOrder places a function-body-only precedent before its dependent', () => {
     const umsatz = sheet('s-umsatz', 'Umsatz', {
       B2: '1200', E1: '3',