style(frontend): look&feel v2 refinements — 70% gate modal, structured cards, mono labels

- ArtifactGateWidget: gate modal 90%→70% of viewport, larger heading, more padding.
- ArtifactView StructuredValue: depth-aware hierarchy (top-level hairline dividers,
  warm bg-muted card surfaces + shadow for object-array items).
- index.css: .thot-label micro-labels switched to the mono register (SF Mono) with
  tightened tracking, distinguishing labels/meta from sans body prose.

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

frontend/src/index.css

@@ -144,11 +144,14 @@
  * Scoped so it never leaks into widgets.
  */
 @layer components {
-  /* Standard micro-label: panel headers, nav sections, meta rows. */
+  /* Standard micro-label: panel headers, nav sections, meta rows. Set in the
+     mono register so labels/meta read as a distinct typographic layer from the
+     sans body prose (mono needs less tracking than the sans did). */
   .thot-label {
+    font-family: var(--font-mono);
     font-size: 0.6875rem;
     font-weight: 600;
-    letter-spacing: 0.12em;
+    letter-spacing: 0.06em;
     text-transform: uppercase;
     color: oklch(var(--muted-foreground));
   }