feat(grid-editor): add manual cell color control via right-click menu

Users can now right-click any cell to set text color (red, green, blue,
orange, purple, black) or remove the color bar without changing text.
A "reset" option restores the OCR-detected color. This enables accurate
Ground Truth marking when OCR assigns colors to wrong cells.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

admin-lehrer/components/grid-editor/useGridEditor.ts

@@ -752,6 +752,39 @@ export function useGridEditor(sessionId: string | null) {
     setSelectedCells(new Set())
   }, [])
 
+  /**
+   * Set a manual color override on a cell.
+   * - hex string (e.g. "#dc2626"): force text color
+   * - null: force no color (clear bar)
+   * - undefined: remove override, restore OCR-detected color
+   */
+  const setCellColor = useCallback(
+    (cellId: string, color: string | null | undefined) => {
+      if (!grid) return
+      pushUndo(grid.zones)
+      setGrid((prev) => {
+        if (!prev) return prev
+        return {
+          ...prev,
+          zones: prev.zones.map((zone) => ({
+            ...zone,
+            cells: zone.cells.map((cell) => {
+              if (cell.cell_id !== cellId) return cell
+              if (color === undefined) {
+                // Remove override entirely — restore OCR behavior
+                const { color_override: _, ...rest } = cell
+                return rest
+              }
+              return { ...cell, color_override: color }
+            }),
+          })),
+        }
+      })
+      setDirty(true)
+    },
+    [grid, pushUndo],
+  )
+
   /** Toggle bold on all selected cells (and their columns). */
   const toggleSelectedBold = useCallback(() => {
     if (!grid || selectedCells.size === 0) return
@@ -881,5 +914,6 @@ export function useGridEditor(sessionId: string | null) {
     clearCellSelection,
     toggleSelectedBold,
     autoCorrectColumnPatterns,
+    setCellColor,
   }
 }