Add cell selection copy/cut/paste with formula reference adjustment

- store: copySelection/cutSelection/pasteClipboard/cancelClipboard hold an
  in-memory (unpersisted) clipboard buffer of raw cell inputs + formatting.
- shiftFormula rewrites every relative cell reference in a copied/cut formula
  by the move delta (the engine has no $-absolute refs, so all refs qualify).
  Copy shifts every reference uniformly; cut only follows references that
  point inside the moved block itself, leaving refs to untouched cells alone
  and never touching a qualified Table!A1 reference.
- paste anchors on a smaller/single-cell selection (and overflows past it),
  or tiles across a selection that's an exact larger multiple of the buffer.
- TableGrid: Strg+C/X/V shortcuts, context-menu entries, a dashed "clip-src"
  source marker, and Escape drops a pending buffer.
- document.spec.ts: coverage for reference shifting, tiled/fill paste, cut's
  inside/outside reference split, format copy, and Escape cancelling.

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

src/components/TableGrid.vue

@@ -118,6 +118,13 @@ const inSelection = (c: number, r: number) =>
 const colHot = (c: number) => selRects.value.some((k) => c >= k.c0 && c <= k.c1);
 const rowHot = (r: number) => selRects.value.some((k) => r >= k.r0 && r <= k.r1);
 
+// ---- clipboard source marker (dashed outline, Excel-style "marching ants") --
+const clipRect = computed(() => store.clipboardRect.value);
+const inClipSource = (c: number, r: number) => {
+  const k = clipRect.value;
+  return !!k && k.tableId === props.tableId && c >= k.c0 && c <= k.c1 && r >= k.r0 && r <= k.r1;
+};
+
 const clampC = (c: number) => Math.max(0, Math.min((table.value?.cols ?? 1) - 1, c));
 const clampR = (r: number) => Math.max(0, Math.min((table.value?.rows ?? 1) - 1, r));
 const focusRoot = () => nextTick(() => root.value?.focus());
@@ -478,6 +485,25 @@ function onKeydown(e: KeyboardEvent) {
   const ext = e.shiftKey;
   const mod = e.ctrlKey || e.metaKey;
 
+  if (mod) {
+    const k = e.key.toLowerCase();
+    if (k === 'c') {
+      e.preventDefault();
+      e.stopPropagation();
+      return store.copySelection();
+    }
+    if (k === 'x') {
+      e.preventDefault();
+      e.stopPropagation();
+      return store.cutSelection();
+    }
+    if (k === 'v') {
+      e.preventDefault();
+      e.stopPropagation();
+      return store.pasteClipboard();
+    }
+  }
+
   switch (e.key) {
     case 'ArrowUp':
       e.preventDefault();
@@ -534,6 +560,7 @@ function onKeydown(e: KeyboardEvent) {
       return store.clearSelectionCells();
     case 'Escape':
       clearFlow();
+      store.cancelClipboard();
       return;
     default:
       if (e.key.length === 1 && !e.ctrlKey && !e.metaKey && !e.altKey) {
@@ -552,13 +579,21 @@ function onContextMenu(e: MouseEvent) {
   if (hit && !inSelection(hit.c, hit.r)) select(hit.c, hit.r);
   menu.value = { x: e.clientX, y: e.clientY, open: true };
 }
-const menuItems = computed(() => [
-  { label: 'Zeile hinzufügen', action: () => store.addRows(props.tableId) },
-  { label: 'Spalte hinzufügen', action: () => store.addColumns(props.tableId) },
-  { label: 'Letzte Zeile entfernen', action: () => store.removeLastRow(props.tableId) },
-  { label: 'Letzte Spalte entfernen', action: () => store.removeLastColumn(props.tableId) },
-  { label: 'Auswahl leeren', action: () => store.clearSelectionCells() },
-]);
+const menuItems = computed(() => {
+  const items = [
+    { label: 'Kopieren', action: () => store.copySelection() },
+    { label: 'Ausschneiden', action: () => store.cutSelection() },
+  ];
+  if (store.hasClipboard.value) items.push({ label: 'Einfügen', action: () => store.pasteClipboard() });
+  items.push(
+    { label: 'Zeile hinzufügen', action: () => store.addRows(props.tableId) },
+    { label: 'Spalte hinzufügen', action: () => store.addColumns(props.tableId) },
+    { label: 'Letzte Zeile entfernen', action: () => store.removeLastRow(props.tableId) },
+    { label: 'Letzte Spalte entfernen', action: () => store.removeLastColumn(props.tableId) },
+    { label: 'Auswahl leeren', action: () => store.clearSelectionCells() },
+  );
+  return items;
+});
 
 // ---- column resize --------------------------------------
 let resize: { col: number; startX: number; startW: number } | null = null;
@@ -674,6 +709,7 @@ defineExpose({
               :class="{
                 'is-active': isActive(c - 1, r - 1),
                 'in-range': inSelection(c - 1, r - 1),
+                'clip-src': inClipSource(c - 1, r - 1),
                 numeric: store.cellIsNumeric(tableId, c - 1, r - 1),
                 error: store.cellIsError(tableId, c - 1, r - 1),
                 frozen: (c - 1 < frozenCols) || (r - 1 < frozenRows),
@@ -824,6 +860,22 @@ defineExpose({
 .cell.is-active {
   box-shadow: inset 0 0 0 2px var(--accent);
 }
+.cell.clip-src {
+  outline: 1px dashed var(--accent);
+  outline-offset: -1px;
+  animation: clip-src-march 0.6s linear infinite;
+}
+@keyframes clip-src-march {
+  from {
+    outline-offset: -1px;
+  }
+  50% {
+    outline-offset: -2px;
+  }
+  to {
+    outline-offset: -1px;
+  }
+}
 .handle {
   position: absolute;
   right: -3px;