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/viewers/ArtifactView.tsx

@@ -109,7 +109,7 @@ function StructuredValue({ value }: { value: unknown }): ReactElement {
     <div className="flex flex-col gap-4">
       {Object.entries(value as Record<string, unknown>).map(([k, v]) => (
         <section key={k}>
-          <h4 className="mb-1.5 text-[0.72rem] font-semibold uppercase tracking-[0.1em] text-muted-foreground">
+          <h4 className="thot-label mb-1.5">
             {humanizeKey(k)}
           </h4>
           <StructuredValue value={v} />