feat: Add Select component for form inputs

[Copilot]

The design system lacked a native <Select> component, forcing consumers to hand-roll styled <select> elements inconsistently. This adds a fully-featured Select component aligned with the existing Input visual contract.

Component API

<Select
  id="model-select"
  value={selectedValue}
  onChange={(value) => setSelectedValue(value)}
  placeholder="Select a model"
  error="Please select a value"
  size="md"
  fullWidth
>
  <Select.Option value="gpt-4">GPT-4</Select.Option>
  <Select.Option value="claude-3">Claude 3</Select.Option>
</Select>

Key design decisions:

• onChange receives string directly (not a ChangeEvent) — matches the expected API from the issue
• error is a string (not boolean) — component owns error message display with aria-describedby wiring
• Placeholder rendered as disabled hidden option — appears in closed state but cannot be re-selected
• Select.Option is a compound sub-component wrapping native <option> for ergonomic usage
• Ref forwarded to the underlying <select> element

Implementation

• Select.tsx — forwardRef wrapper around native <select>, compound with Select.Option
• Select.module.css — Reuses --lufa-component-input-* tokens for visual parity with Input; size variants (sm/md/lg) use the same padding/font-size token ladder as the input token set
• Exports: Select, SelectProps, SelectSizeValue

Coverage

• Playwright CT: a11y, rendering, all states (error, disabled, fullWidth, sizes), onChange value delivery, aria-describedby association, visual regression snapshots
• Storybook: Default, Placeholder, States, Size, FullWidth stories
• Docusaurus: live demos + full props reference