feat(frontend): skeleton loading states, empty-state glow, light-mode polish

- Add shared skeleton building blocks (SkeletonHero, SkeletonMetricRow,
  SkeletonPanel, SkeletonRows) and wire them into every view's initial
  loading branch, replacing bare spinners for a cohesive shimmering shell.
- Polish EmptyState with a glow-ring icon chip instead of a flat icon.
- Harden .light theme: color-scheme, tuned scrollbar + selection for pale
  canvas. Dark/light theme toggle (next-themes) already persisted in TopBar.

services/frontend/src/app/globals.css

@@ -53,6 +53,7 @@
 
 /* ── Light theme overrides ── */
 .light {
+  color-scheme: light;
   --color-canvas: oklch(0.96 0.012 80);
   --color-canvas-2: oklch(0.92 0.014 80);
   --color-surface: oklch(1 0 0 / 0.7);
@@ -65,6 +66,18 @@
   --color-hairline: oklch(0.22 0.02 70 / 0.12);
 }
 
+/* Light-theme scrollbar + selection tuned for pale canvas */
+.light ::-webkit-scrollbar-thumb {
+  background: oklch(0.22 0.02 70 / 0.2);
+}
+.light ::-webkit-scrollbar-thumb:hover {
+  background: oklch(0.22 0.02 70 / 0.34);
+}
+.light ::selection {
+  background: var(--color-signal-glow);
+  color: var(--color-ink);
+}
+
 @layer base {
   * {
     @apply border-transparent;