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/MultiselectWidget.tsx

@@ -21,7 +21,7 @@ export function MultiselectWidget({ descriptor, onRespond }: WidgetProps) {
   const isDisabled = descriptor.allow_empty === false && checked.size === 0;
 
   return (
-    <div className="space-y-4">
+    <div className="space-y-4 rounded-xl border border-border/70 bg-card p-4 shadow-sm">
       {descriptor.title && (
         <p className="text-[0.95rem] font-semibold leading-snug text-foreground">
           {descriptor.title}
@@ -46,7 +46,9 @@ export function MultiselectWidget({ descriptor, onRespond }: WidgetProps) {
           {options.map((o) => (
             <label
               key={o.id}
-              className="flex cursor-pointer items-center gap-2.5 rounded-lg border border-transparent px-2.5 py-2 text-sm text-foreground transition-colors hover:bg-accent"
+              className={`flex cursor-pointer items-center gap-2.5 rounded-lg border px-2.5 py-2 text-sm text-foreground transition-colors hover:bg-accent ${
+                checked.has(o.id) ? "border-border/60 bg-accent/60" : "border-transparent"
+              }`}
             >
               <input
                 type="checkbox"
@@ -61,7 +63,7 @@ export function MultiselectWidget({ descriptor, onRespond }: WidgetProps) {
       </div>
       <div>
         <button
-          className="rounded-lg bg-primary px-4 py-2 text-sm font-medium text-primary-foreground transition-colors hover:bg-[oklch(var(--primary-hover))] disabled:cursor-not-allowed disabled:opacity-50"
+          className="rounded-md bg-primary px-4 py-2 text-sm font-semibold text-primary-foreground shadow-xs transition-colors hover:bg-[oklch(var(--primary-hover))] disabled:cursor-not-allowed disabled:opacity-50"
           disabled={isDisabled}
           onClick={() => onRespond({ id: descriptor.id, kind: "multiselect", choices: Array.from(checked) })}
         >