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/NavSessions.tsx

@@ -39,7 +39,7 @@ export function NavSessions({ sessions, activeSessionId, onOpenPanel, menuFor }:
                 }
               }}
               className={[
-                "group flex w-full cursor-pointer items-center gap-2 rounded-md px-2 py-1 text-left transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-ring/40",
+                "group flex w-full cursor-pointer items-center gap-2 rounded-lg px-2 py-1 text-left transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-ring/40",
                 // The in-progress (active) session is marked in yellow — both its dot
                 // and its row background — so it stands out from the rest of the rail.
                 active ? "bg-[oklch(var(--warning)/0.22)]" : "hover:bg-accent",