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));
   }

frontend/src/viewers/ArtifactView.tsx

@@ -77,8 +77,10 @@ function humanizeKey(k: string): string {
 
 /** Renders any artifact payload as readable text — NEVER raw JSON. Objects become
  *  labelled sections, arrays become bullet lists, primitives become paragraphs;
- *  strings render as markdown. Nested structures recurse. */
-function StructuredValue({ value }: { value: unknown }): ReactElement {
+ *  strings render as markdown. Nested structures recurse. `depth` drives the
+ *  visual hierarchy: top-level sections are split by hairline rules, nested fields
+ *  sit tighter, and object-arrays become grouped cards. */
+function StructuredValue({ value, depth = 0 }: { value: unknown; depth?: number }): ReactElement {
   if (value === null || value === undefined || value === "") {
     return <p className="text-sm text-muted-foreground">—</p>;
   }
@@ -98,21 +100,32 @@ function StructuredValue({ value }: { value: unknown }): ReactElement {
     return (
       <div className="flex flex-col gap-3">
         {value.map((item, i) => (
-          <div key={i} className="rounded-lg border border-border/60 p-3">
-            <StructuredValue value={item} />
+          <div
+            key={i}
+            className="rounded-xl border border-border/70 bg-muted/50 p-4 shadow-xs"
+          >
+            <StructuredValue value={item} depth={depth + 1} />
           </div>
         ))}
       </div>
     );
   }
+  const entries = Object.entries(value as Record<string, unknown>);
   return (
-    <div className="flex flex-col gap-4">
-      {Object.entries(value as Record<string, unknown>).map(([k, v]) => (
-        <section key={k}>
-          <h4 className="thot-label mb-1.5">
-            {humanizeKey(k)}
-          </h4>
-          <StructuredValue value={v} />
+    <div className="flex flex-col">
+      {entries.map(([k, v], i) => (
+        <section
+          key={k}
+          className={
+            i === 0
+              ? undefined
+              : depth === 0
+                ? "mt-5 border-t border-border/50 pt-5"
+                : "mt-3"
+          }
+        >
+          <h4 className="thot-label mb-1.5">{humanizeKey(k)}</h4>
+          <StructuredValue value={v} depth={depth + 1} />
         </section>
       ))}
     </div>

frontend/src/widgets/ArtifactGateWidget.tsx

@@ -7,7 +7,7 @@ import { ReservedControls } from "./ReservedControls";
 import { LinkageHost } from "./LinkageHost";
 
 /**
- * Artifact review gate rendered as a full-screen (90%) modal: the artifact fills
+ * Artifact review gate rendered as a centered (70%) modal: the artifact fills
  * the top (scrollable) area via ArtifactView; the action bar (options + reserved
  * controls) sits at the bottom. The gate contract forbids silent dismissal, so the
  * dialog has no close button and is not closeable by Esc/backdrop — the only way
@@ -30,11 +30,13 @@ export function ArtifactGateWidget({ descriptor, onRespond, sessionId }: WidgetP
     <Dialog open>
       <DialogContent
         showCloseButton={false}
-        className="grid h-[90vh] w-[90vw] max-w-[90vw] grid-rows-[auto_1fr_auto] gap-3 sm:max-w-[90vw]"
+        className="grid h-[70vh] w-[70vw] max-w-[70vw] grid-rows-[auto_1fr_auto] gap-3.5 sm:max-w-[70vw]"
       >
-        <DialogTitle>{descriptor.title ?? "Artifact review"}</DialogTitle>
+        <DialogTitle className="text-lg leading-snug">
+          {descriptor.title ?? "Artifact review"}
+        </DialogTitle>
 
-        <div className="min-h-0 overflow-auto rounded-xl border border-border/70 bg-background p-3">
+        <div className="min-h-0 overflow-auto rounded-xl border border-border/70 bg-background p-5">
           {descriptor.artifact ? (
             <GateArtifactBody artifact={descriptor.artifact} sessionId={sessionId} />
           ) : (