Keep the first-selected cell marked as active during a multi-cell selection

The active cell (bold border/handle, formula bar target) used to track
selection.focus, so it followed the mouse/cursor while dragging or
shift-extending a range. Switch it to selection.anchor -- the first cell of
the current selection action -- so it stays put while the range grows,
matching Excel/Numbers. Tab/Enter data-entry flow already keeps anchor ==
focus while it moves the cursor, so that behaviour is unaffected.

- document.ts: activeCell/activeCellName/activeCellInput read the anchor.
- FormulaBar.vue: commits to the anchor cell to match what it displays.
- TableGrid.vue: isActive() (border + handle) and F2/type-to-edit target
  the anchor; Home/End keep using the cursor's row via s.focus.

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

src/store/document.ts

@@ -993,18 +993,21 @@ function createStore() {
     });
   }
 
-  const activeCell = computed<CellCoord>(() => doc.selection?.focus ?? { col: 0, row: 0 });
+  // The "active cell" is always the anchor -- the first cell of the current
+  // selection action (click, shift-click, drag start, ...) -- so it stays put
+  // while a multi-cell selection is being made, matching Excel/Numbers.
+  const activeCell = computed<CellCoord>(() => doc.selection?.anchor ?? { col: 0, row: 0 });
 
   const activeCellName = computed(() => {
     const sel = doc.selection;
     const t = sel ? tableById(sel.tableId) : activeTable.value;
-    const cell = sel?.focus ?? { col: 0, row: 0 };
+    const cell = sel?.anchor ?? { col: 0, row: 0 };
     return `${t?.name ?? '—'}::${toA1(cell.col, cell.row)}`;
   });
 
   const activeCellInput = computed(() => {
     const sel = doc.selection;
-    return sel ? cellInput(sel.tableId, sel.focus.col, sel.focus.row) : '';
+    return sel ? cellInput(sel.tableId, sel.anchor.col, sel.anchor.row) : '';
   });
 
   const selectionAggregate = computed(() => {