fix(app): hide prompt placeholder for whitespace input

[ShrootBuck]

Issue for this PR

Closes #28099

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The prompt composer treated whitespace-only input as empty while reconciling editor state. That kept prompt.dirty() false, so the placeholder overlay stayed visible after typing spaces.

This updates the reset check to distinguish typed whitespace from browser-generated empty contenteditable line breaks. Spaces now hide the placeholder, while deleting everything resets the prompt so the placeholder comes back. Whitespace-only prompts remain unsendable because submit still uses the existing trim-based empty prompt guard.

How did you verify your code works?

• Ran bun test --preload ./happydom.ts ./src/components/prompt-input from packages/app
• Ran bun typecheck from packages/app
• Pre-push hook ran bun turbo typecheck successfully
• Started bun run --cwd packages/desktop dev and checked the prompt behavior locally

Screenshots / recordings

Before: typing spaces leaves the placeholder visible over the caret.

After: typing spaces hides the placeholder; deleting all content restores it. Submit remains disabled for whitespace-only input.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍