Show the selected range, not just the active cell, in status/formula bar

New store computed selectionRangeName: for a single cell it's identical
to activeCellName ("Umsatz::B2"), for a multi-cell selection it shows
the full primary rectangle ("Umsatz::B2:E6"), and a discontiguous
multi-selection gets a "+N" suffix for the further rectangles.
activeCellName itself is untouched (still anchor-only, since that's what
a formula/edit actually targets) -- only the formula bar's name field and
the status bar's range label switch to the new computed.

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

src/store/__tests__/document.spec.ts

@@ -138,6 +138,22 @@ describe('seed document acceptance (canvas model)', () => {
     // the active cell is the anchor -- the first cell of the selection -- not
     // wherever the selection was extended to.
     expect(s.activeCellName.value).toBe(`Umsatz::${toA1(1, 1)}`);
+    // selectionRangeName shows the full range instead (status bar / formula bar)
+    expect(s.selectionRangeName.value).toBe(`Umsatz::${toA1(1, 1)}:${toA1(1, 5)}`);
+  });
+
+  it('selectionRangeName shows a single address for a 1x1 selection and a "+N" suffix for a discontiguous multi-selection', () => {
+    const s = fresh();
+    const umsatz = tableId(s, 'Umsatz');
+    s.setSelection({ tableId: umsatz, anchor: { col: 0, row: 0 }, focus: { col: 0, row: 0 } });
+    expect(s.selectionRangeName.value).toBe(`Umsatz::${toA1(0, 0)}`);
+
+    s.addToSelection(umsatz, 3, 3);
+    s.addToSelection(umsatz, 5, 5);
+    // addToSelection() moves the primary rect to the just-clicked cell (see
+    // "Mehrfachauswahl" tests below) -- the range name follows it, plus a
+    // "+N" for the other rectangles now in the selection.
+    expect(s.selectionRangeName.value).toBe(`Umsatz::${toA1(5, 5)} +2`);
   });
 
   it('recalculation visits precedents before dependents across tables', () => {