feat(ui): refine gem-blue acrylic palette and focus borders

Derive the gem-blue palette with OKLCH tonal and complementary-color
rules while preserving acrylic opacity, blur and surface hierarchy.

Darker focus tokens exposed stacked field rings, fallback outlines and
persistent basemap selection rings. Keep fallback focus styles in the
base layer and use one themed field boundary or inset card stroke.
Preserve keyboard focus, invalid states and native input semantics.

Add browser regression coverage for pointer and keyboard focus, all
basemap presentations, narrow layouts and rendered contrast.

Validation in an isolated checkout: pnpm lint, pnpm typecheck,
pnpm test (393 tests), focused browser suites and pnpm build.

src/features/agent/components/agent-persona.tsx

@@ -44,7 +44,7 @@ function useElementVisibility() {
 
 function PersonaFallback() {
   return (
-    <span className="grid h-full w-full place-items-center rounded-full border border-slate-300/70 bg-slate-100 text-slate-500">
+    <span className="grid h-full w-full place-items-center rounded-full border border-[var(--surface-border)]/70 bg-[var(--surface-control)] text-[var(--ink-muted)]">
       <Bot className="h-2/5 w-2/5" aria-hidden="true" />
     </span>
   );