Rethink facility grid UI: spreadsheet-style with row/col/all selectors

Replace the broken flex+grid layout with a proper CSS Grid that includes
header gutters for row/column selection buttons. Cells now fill naturally.
Corner button selects all, row/col headers toggle-select entire rows/columns.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

resources/css/app.css

@@ -92,13 +92,19 @@
     }
 
     .facility-cell--selected {
-        background-color: oklch(0.65 0.2 250 / 0.2);
-        box-shadow: inset 0 0 0 2.5px oklch(0.55 0.2 250 / 0.7);
+        background-color: oklch(0.92 0.06 250) !important;
+        box-shadow: inset 0 0 0 2px oklch(0.60 0.18 250 / 0.6);
     }
 
     .facility-cell--drop-target {
-        background-color: oklch(0.7 0.15 155 / 0.2);
-        box-shadow: inset 0 0 0 2.5px oklch(0.55 0.15 155 / 0.6);
+        background-color: oklch(0.92 0.06 155) !important;
+        box-shadow: inset 0 0 0 2px oklch(0.55 0.15 155 / 0.6);
+        animation: pulse-drop 1s ease-in-out infinite;
+    }
+
+    @keyframes pulse-drop {
+        0%, 100% { opacity: 1; }
+        50% { opacity: 0.7; }
     }
 }