feat(chat): add arrow key navigation for thinking mode in model selector (#769)

* feat(chat): add arrow key navigation for thinking mode in model selector

Adds keyboard arrow key navigation for thinking mode selection within the model picker, enabling a fully keyboard-driven workflow for model and variant selection.

Changes:
- Model Selector: Added ←/→ arrow key handling to cycle thinking variants while picker is open
- Introduced transient state to track per-model variant selections during picker lifecycle
- Updated metadata display logic to show thinking mode only after adjustment with arrow keys
- Added contextual footer hints that appear only when highlighted model supports thinking variants
- Modified Enter key behavior to apply both selected model and pending thinking variant together
- Help Dialog: Added in-picker navigation shortcuts (↑↓ navigate, ←→ adjust thinking)
- Documentation: Updated CHANGELOG.md files with feature entry

Behavior:
- Navigate models with ↑↓, adjust thinking with ←→, press Enter to apply both
- Footer hints update dynamically based on highlighted model capabilities
- Thinking display persists only for models adjusted with arrow keys
- No-op for models without variants (clean, no feedback)

Testing:
- Type-check, lint, and build all pass
- Manual testing verified on dev server

packages/ui/src/components/chat/MarkdownRenderer.tsx

@@ -1279,7 +1279,7 @@ export const MarkdownRenderer: React.FC<MarkdownRendererProps> = ({
   variant = 'assistant',
   onShowPopup,
 }) => {
-  const { files, editor, runtime } = useRuntimeAPIs();
+  const { editor, runtime } = useRuntimeAPIs();
   const streamdownContainerRef = React.useRef<HTMLDivElement>(null);
   const effectiveDirectory = useEffectiveDirectory() ?? '';
   const mermaidBlocks = React.useMemo(() => extractMermaidBlocks(content), [content]);
@@ -1354,7 +1354,7 @@ export const SimpleMarkdownRenderer: React.FC<{
   onShowPopup,
   allowMermaidWheelZoom = false,
 }) => {
-  const { files, editor, runtime } = useRuntimeAPIs();
+  const { editor, runtime } = useRuntimeAPIs();
   const renderedContent = React.useMemo(
     () => (stripFrontmatter ? stripLeadingFrontmatter(content) : content),
     [content, stripFrontmatter],