style(frontend): look&feel v2 — layered shadows, sharper radii, mono data register

Colors (GSD palette) untouched. New visual vocabulary across the whole GUI:
- warm-tinted layered shadow tokens (--shadow-xs/sm/md) replacing flat rings;
  cards/dialogs framed with hairline border + real elevation
- strict radii scale: xl cards / lg inputs / md controls / 2xl dialogs;
  buttons move from pills to rounded-md
- third type register: --font-mono + tabular-nums on SQL, timers, dates, ids;
  standardized .thot-label micro-label tier
- prose: framed tables with uppercase headers, elevated code blocks
- SqlViewer badges moved from hardcoded Tailwind greens/reds to success/
  destructive token tints; ResultsPanel inline styles replaced with classes
- artifact gate: first option rendered as filled primary action

Verified offline via Playwright against a mock backend replaying a real psd
session (no VPN needed). tsc clean, 115/115 vitest.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/widgets/ArtifactWidget.tsx

@@ -10,7 +10,7 @@ export function ArtifactWidget({ descriptor }: WidgetProps) {
     <div className="space-y-2">
       {descriptor.title && <p className="font-medium">{descriptor.title}</p>}
       {descriptor.artifact?.content && (
-        <pre className="overflow-auto max-h-64 bg-muted p-3 text-sm rounded border">
+        <pre className="overflow-auto max-h-64 bg-muted p-3 text-sm font-mono rounded-lg border border-border/70 shadow-xs">
           {descriptor.artifact.content}
         </pre>
       )}