Fix composer placeholder selection - #107

Summary

• Hide the composer placeholder mirror while native textarea selection is active so selected text is not drawn twice.
• Select the first bracket placeholder after inserting an expert prompt template so users can immediately type the replacement value.
• Select the whole bracket placeholder when the caret lands inside it, making one-click replacement easier.

Validation

• pnpm exec vitest run src/components/chat-composer.test.ts src/components/chat-panel.test.ts --reporter=verbose
• pnpm exec eslint src/components/chat-composer.tsx src/components/chat-composer.test.ts
• pnpm exec tsc --noEmit --pretty false
• git diff --check origin/staging...HEAD
• pnpm build