Snap-to-grid for moving/resizing canvas objects, plus a per-sheet grid size

Sheet.gridSize (4-200px, default 24, configurable in Settings -> Blatt)
now doubles as both the visible dot-grid's spacing and the snap
increment while dragging to move any canvas object, or free/viewport-
resizing a chart/text/button/fixed-viewport table. An "Automatisch"
table's resize is left unsnapped on top of this -- it already
discretizes to row/col boundaries by construction.

Also fixes the visible grid not actually lining up with real snap
positions once zoomed: the dot pattern's background-size now scales
with zoom like the objects themselves already did, instead of staying a
fixed screen-pixel size regardless of zoom level.

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

src/store/__tests__/document.spec.ts

@@ -297,6 +297,21 @@ describe('per-sheet appearance, zoom & CSV default (saved in the file)', () => {
     void b;
   });
 
+  it('grid size (snap increment) is per-sheet, clamped and rounded', () => {
+    const s = fresh();
+    const a = s.doc.sheets[0].id;
+    const b = s.doc.sheets[1].id;
+    expect(s.doc.sheets[0].gridSize).toBeUndefined(); // unset = DEFAULT_GRID_SIZE at use-sites
+    s.setSheetGridSize(a, 40.6);
+    expect(s.doc.sheets[0].gridSize).toBe(41); // rounded
+    expect(s.doc.sheets[1].gridSize).toBeUndefined(); // untouched
+    s.setSheetGridSize(a, 1); // below MIN_GRID_SIZE
+    expect(s.doc.sheets[0].gridSize).toBe(4);
+    s.setSheetGridSize(a, 9999); // above MAX_GRID_SIZE
+    expect(s.doc.sheets[0].gridSize).toBe(200);
+    void b;
+  });
+
   it('zoom is stored per sheet; the active-sheet zoom follows the tab', () => {
     const s = fresh();
     expect(s.activeZoom.value).toBe(1);
@@ -310,26 +325,30 @@ describe('per-sheet appearance, zoom & CSV default (saved in the file)', () => {
     expect(s.activeZoom.value).toBe(1.5); // sheet 1 kept its own
   });
 
-  it('appearance + zoom + csvDelimiter survive a JSON round-trip; bad values dropped', () => {
+  it('appearance + zoom + gridSize + csvDelimiter survive a JSON round-trip; bad values dropped', () => {
     const s = fresh();
     const a = s.doc.sheets[0].id;
     s.setSheetBackground(a, '#abcdef');
     s.setSheetForeground(a, '#012');
     s.setSheetShowGrid(a, false);
     s.setSheetZoom(a, 1.25);
+    s.setSheetGridSize(a, 32);
     s.setCsvDelimiter(';');
     const before = s.snapshot();
     s.replaceDocument(parseDocument(serializeDocument(before)));
     expect(s.snapshot()).toEqual(before);
     expect(s.doc.csvDelimiter).toBe(';');
+    expect(s.doc.sheets[0].gridSize).toBe(32);
 
     const doc = JSON.parse(serializeDocument(s.snapshot()));
     doc.sheets[0].background = 'not-a-color';
     doc.sheets[0].zoom = 99;
+    doc.sheets[0].gridSize = 'huge';
     doc.csvDelimiter = 'x';
     s.replaceDocument(parseDocument(JSON.stringify(doc)));
     expect(s.doc.sheets[0].background).toBeUndefined();
     expect(s.doc.sheets[0].zoom).toBe(2); // clamped, not dropped
+    expect(s.doc.sheets[0].gridSize).toBeUndefined(); // not a number -> dropped
     expect(s.doc.csvDelimiter).toBe(',');
   });
 });