feat: Add single selection mode with search functionality

[pkuliga]

🚀 Feature Request: Single Selection Mode

Problem

The current MultiSelect component is designed exclusively for multi-selection. However, there are use cases where a searchable single-select is needed — something a basic <select> or shadcn Select component doesn't provide out of the box.

Proposed Solution

Add a singleSelect (or mode: "single" | "multi") prop that:

• Limits selection to one item at a time
• Keeps the search/filter functionality intact
• Automatically closes the popover after selection (like closeOnSelect)
• Clears the previous selection when a new item is picked
• Still supports all existing props: searchable, placeholder, disabled, emptyIndicator, etc.

Example Usage

<MultiSelect
  options={options}
  onValueChange={setSelected}
  singleSelect={true}
  placeholder="Search and select one..."
/>

Why Not Use shadcn Select?

The native shadcn Select component lacks built-in search/filter functionality, making it impractical for large option lists. This component already has the search infrastructure — a singleSelect mode would reuse it elegantly.

Acceptance Criteria

• New singleSelect?: boolean prop (default: false)
• Selecting an item replaces the previous selection
• Popover closes automatically on selection when singleSelect={true}
• onValueChange still returns string[] for API compatibility (with a single element)
• Existing multi-select behaviour is unaffected