[FIX] dashboard: hide overflow area after snappy resize

Before this commit:
- The canvas was sized from the global grid instead of the overlay,
  causing visible white margins after the snappy resize (commit 3c8df4b).

After this commit:
- The dashboard applies overflow-hidden to clip the extra painted area,
  keeping the snappy resizing behavior.

Task: 5223156

Author: rmbh-odoo

src/components/dashboard/dashboard.xml

@@ -1,7 +1,10 @@
 <templates>
   <t t-name="o-spreadsheet-SpreadsheetDashboard">
     <div class="o-grid o-two-columns" t-ref="dashboard" tabindex="-1" t-on-wheel="onMouseWheel">
-      <div class="mx-auto h-100 position-relative" t-ref="grid" t-att-style="gridContainer">
+      <div
+        class="mx-auto h-100 position-relative overflow-hidden"
+        t-ref="grid"
+        t-att-style="gridContainer">
         <GridOverlay
           onGridResized.bind="onGridResized"
           onGridMoved.bind="moveCanvas"

tests/grid/__snapshots__/dashboard_grid_component.test.ts.snap

@@ -6,7 +6,7 @@ exports[`Grid component in dashboard mode simple dashboard rendering snapshot 1`
   tabindex="-1"
 >
   <div
-    class="mx-auto h-100 position-relative"
+    class="mx-auto h-100 position-relative overflow-hidden"
     style="max-width:2496px; "
   >
     <div