[autocomplete] Improve command palette accessibility

[SSDWGG]

Summary

• focus the command palette autocomplete input when the dialog opens
• give the input an explicit accessible name and describe the available keyboard commands
• make the visible Actions shortcut platform-neutral by showing Ctrl/Cmd + K

Why

The command palette demo currently opens with the inline autocomplete list already active, and the footer shortcuts are only visible text. Screen reader users can miss the search input and the available command shortcuts. This updates both CSS Modules and Tailwind variants so the demo announces the input and exposes the instructions through aria-describedby.

Closes #4247
Closes #4248

Tests

• pnpm prettier
• pnpm typescript
• pnpm exec eslint docs/src/app/(docs)/react/components/autocomplete/demos/command-palette/css-modules/index.tsx docs/src/app/(docs)/react/components/autocomplete/demos/command-palette/tailwind/index.tsx --max-warnings 0
• pnpm stylelint docs/src/app/(docs)/react/components/autocomplete/demos/command-palette/css-modules/index.module.css

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4949
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4949
  

commit: ae7b7f3

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	0B(0.00%)	0B(0.00%)

Details of bundle changes

Performance

Total duration: 1,244.68 ms -143.01 ms^(-10.3%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,858.77 ms -239.06 ms^(-11.4%)

Test	Duration	Renders
Checkbox mount (500 instances)	72.09 ms ▼-19.59 ms(-21.4%)	1 (+0)

11 tests within noise — details

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[SSDWGG]

The code/test checks are green; the remaining failing check is test-label-applied, which requires at least one PR label. I do not have permission to add labels from this fork; could a maintainer please add accessibility when reviewing?

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve touch focus behavior when opening

When the palette is opened from a touch trigger, this unconditional initialFocus={inputRef} bypasses Dialog.Popup's default touch behavior in DialogPopup.tsx, which focuses the popup instead of an input to avoid opening the virtual keyboard. On phones/tablets, tapping “Open command palette” in this demo (and the Tailwind variant with the same prop) will now immediately focus the search field and show the keyboard; consider using an initialFocus function that only returns the input for non-touch opens and preserves the popup/default behavior for touch.

Useful? React with 👍 / 👎.