Hide the grand-total column group in rollup mode

Rollup mode emits a grand-total column group alongside the subtotals. The
subtotals are the point -- they are what per-branch collapse needs -- but
the grand total sums across the split, and when the split is Prior Year /
Plan / Forecast that sum is meaningless while looking exactly like a real
figure to anyone scanning the sheet.

The engine cannot separate the two: t_totals is { BEFORE, HIDDEN, AFTER },
and in a rollup the grand total *is* the root of the hierarchy that produces
the subtotals. Turning it off turns the subtotals off with it. So it is
hidden rather than suppressed -- the view still computes the column, it is
simply not painted, and collapsed to zero width so there is no gap where it
was.

Targeted by psp-split-total, excluding psp-split-subtotal, which is how the
datagrid already distinguishes them. That survives adding a measure or
rearranging the pivot; hiding by column position would not -- the group
spans one column per measure in `columns`.

Injected into the plugin's shadow root, since a document stylesheet cannot
reach inside it, and re-asserted on config updates because the plugin
element is replaced when the plugin changes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/src/views/Forecast.jsx

@@ -659,6 +659,7 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
       // the named table in time.
       await viewer.load(tableRef.current)
       viewer.setAttribute('theme', dark ? 'Pro Dark' : 'Pro Light')
+      hideSplitTotal()
 
       // restore last-used layout or build default
       // Strip cfg.table — table is already loaded by reference above; a stale name
@@ -705,6 +706,8 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
           const live = await viewer.save()
           adoptSplit(live.split_by || [],
                      live.split_by_depth != null ? live.split_by_depth - 1 : null)
+          // the plugin element is replaced when the plugin changes, so re-assert
+          hideSplitTotal()
           const cfg = await captureConfig()
           if (cfg) await persistLayout(vid, cfg)
         } catch {}
@@ -809,6 +812,46 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
     }
   }
 
+  // In rollup mode the datagrid emits a grand-total column group as well as the
+  // subtotals. The subtotals are the point -- they are what per-branch collapse
+  // needs -- but the grand total sums across the split, and when the split is
+  // Prior Year / Plan / Forecast that sum is meaningless and reads as a real
+  // figure to anyone scanning the sheet.
+  //
+  // The engine cannot separate them: t_totals is { BEFORE, HIDDEN, AFTER }, and in
+  // a rollup the grand total *is* the root of the hierarchy that produces the
+  // subtotals. So it is hidden rather than suppressed -- the view still computes
+  // it, it is simply not painted.
+  //
+  // psp-split-total marks it and psp-split-subtotal marks the subtotals, so this
+  // survives adding measures or rearranging the pivot, unlike hiding by position.
+  // The rule has to live inside the plugin's shadow root; a document stylesheet
+  // cannot reach it.
+  const HIDE_SPLIT_TOTAL_CSS = `
+    th.psp-split-total:not(.psp-split-subtotal),
+    td.psp-split-total:not(.psp-split-subtotal) {
+      visibility: hidden !important;
+      width: 0 !important;
+      min-width: 0 !important;
+      max-width: 0 !important;
+      padding: 0 !important;
+      border-left-width: 0 !important;
+      border-right-width: 0 !important;
+      overflow: hidden !important;
+    }
+  `
+
+  function hideSplitTotal() {
+    const viewer = viewerRef.current
+    const root = viewer?.shadowRoot
+      ?.querySelector('perspective-viewer-datagrid')?.shadowRoot
+    if (!root || root.getElementById?.('pf-hide-split-total')) return
+    const style = document.createElement('style')
+    style.id = 'pf-hide-split-total'
+    style.textContent = HIDE_SPLIT_TOTAL_CSS
+    root.appendChild(style)
+  }
+
   // Row depth is a ViewConfig field, so it is set through restore() rather than
   // by calling set_depth() on the view. That is the whole difference: the config
   // is what the viewer rebuilds its view *from*, so the depth survives every