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/shell/ElapsedTimer.tsx

@@ -36,7 +36,7 @@ export function ElapsedTimer({ running }: { running: boolean }) {
   const live = running && runStartRef.current != null ? Date.now() - runStartRef.current : 0;
   return (
     <span
-      className="shrink-0 tabular-nums text-xs text-muted-foreground"
+      className="shrink-0 font-mono tabular-nums text-xs text-muted-foreground"
       aria-label="Processing time"
       title="Processing time (excludes time spent waiting on the reviewer)"
     >