feat(grid): refine grid page chrome (refined dense-pro, theme kept)

Redesign the grid page shell while preserving the Slate/teal Apple theme:
- header reworked into a branded bar: teal accent marker + uppercase
  eyebrow + supplier name as the hero, with "Magasin" and "Références"
  metric pills
- loading/error state shown as a teal (or error-red) accent chip
- "Valider" save button restyled with the brand gradient + soft shadow
- grid container radius/shadow aligned to the apple-card system

No change to table cell logic or the network columns.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/features/grid/components/heatmap-grid.tsx

@@ -805,11 +805,11 @@ export function HeatmapGrid({ onSelectionChange, isAdmin }: HeatmapGridProps) {
 
             <div
                 ref={tableContainerRef}
-                className="h-full w-full overflow-auto rounded-[12px] relative"
+                className="h-full w-full overflow-auto rounded-[14px] relative"
                 style={{
                     background: "var(--bg-surface)",
                     border: "1px solid var(--border)",
-                    boxShadow: "var(--shadow-sm)"
+                    boxShadow: "var(--shadow-md)"
                 }}
             >
                 <table className="text-sm block" style={{ width: "100%", minWidth: totalWidth }}>