fix: add visually hidden DialogTitle for accessibility

[jaredm563]

Description

Fixes #393

This PR adds a visually hidden DialogTitle to the Command.Dialog component to satisfy Radix UI's accessibility requirements.

Problem

The Command.Dialog component uses @radix-ui/react-dialog internally but doesn't include a DialogTitle, which causes accessibility warnings in React 18+ and testing environments:

Warning: `DialogContent` requires a `DialogTitle` for the component to be accessible for screen reader users.

Solution

Added a single line with a visually hidden DialogTitle that uses the label prop (with a fallback to "Command Menu"):

<RadixDialog.Title style={srOnlyStyles}>{props.label || 'Command Menu'}</RadixDialog.Title>

Benefits

• ✅ Eliminates console warnings in development and test environments
• ✅ Improves accessibility for screen reader users
• ✅ Follows Radix UI's accessibility best practices
• ✅ Maintains current visual design (title is visually hidden)
• ✅ No breaking changes - uses existing label prop
• ✅ Uses existing srOnlyStyles constant already in the codebase

Testing

Developers can now remove workarounds like:

// No longer needed!
const originalConsoleError = console.error;
beforeAll(() => {
  console.error = (...args: unknown[]) => {
    const message = String(args[0]);
    if (message.includes('`DialogContent` requires a `DialogTitle`')) {
      return;
    }
    originalConsoleError(...args);
  };
});

[jaredm563]

I totally understand the 'unstyled / headless' philosophy of the library and that yall probably were intending to keep the DOM tree minimal. BUT, the most recent versions of Radix UI now strictly enforce the presence of a DialogTitle component.

So, without this, developers using cmdk in React 18+ or modern testing environments (like Vitest/Jest with @testing-library) are hit with persistent console.error warnings that can't be resolved without messy workarounds in the consuming codebase.

Soooo this is a pretty small change only intended to satisfy the Radix UI accessibility contract and get rid of those console errors -- and it will still be visually identical for all users.

[aitchiss]

Nice! Would love to get this merged - having a proper title for the dialog component is a must for accessibility; seems like a quick win with a big impact 🙌