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/shared/ai-elements/code-block.tsx

@@ -524,7 +524,7 @@ export const CodeBlockLanguageSelectorTrigger = ({
 }: CodeBlockLanguageSelectorTriggerProps) => (
   <SelectTrigger
     className={cn(
-      "h-7 border-none bg-transparent px-2 text-xs shadow-none",
+      "form-field-inset h-7 border-none bg-transparent px-2 text-xs shadow-none",
       className
     )}
     {...props}

src/shared/ai-elements/prompt-input.tsx

@@ -1276,7 +1276,7 @@ export const PromptInputSelectTrigger = ({
 }: PromptInputSelectTriggerProps) => (
   <SelectTrigger
     className={cn(
-      "border-none bg-transparent font-medium text-muted-foreground shadow-none transition-colors",
+      "form-field-inset border-none bg-transparent font-medium text-muted-foreground shadow-none transition-colors",
       "hover:bg-accent hover:text-foreground aria-expanded:bg-accent aria-expanded:text-foreground",
       className
     )}