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/tailwind.config.ts

@@ -53,9 +53,16 @@ export default {
         md: "calc(var(--radius) - 2px)",
         sm: "calc(var(--radius) - 4px)",
       },
+      boxShadow: {
+        xs: "var(--shadow-xs)",
+        sm: "var(--shadow-sm)",
+        DEFAULT: "var(--shadow-sm)",
+        md: "var(--shadow-md)",
+      },
       fontFamily: {
         sans: ["var(--font-sans)", "sans-serif"],
         heading: ["var(--font-heading)", "sans-serif"],
+        mono: ["var(--font-mono)", "monospace"],
       },
     },
   },