Fix style tag accumulation when loading dialogs

[Copilot]

Style tags were accumulating in document.head each time a dialog was loaded, causing the style sheet to grow indefinitely.

Root Cause

In loadScreen(), old style tags were removed only for regular scenes:

if (!isDialog) {
  // Remove old style tags
  const styleTags = document.head.getElementsByTagName('style')
  for (const styleTag of styleTags) {
    document.head.removeChild(styleTag)
  }
}
// But always append new style tag
const styleElement = doc.head.getElementsByTagName('style')[0]
document.head.appendChild(styleElement)  // ← Appends every time

Changes

• Moved style tag insertion inside the !isDialog block
• Dialogs now inherit parent page styles without injecting duplicates
• Added null check for templates without style tags

if (!isDialog) {
  // Remove old style tags
  const styleTags = document.head.getElementsByTagName('style')
  for (const styleTag of styleTags) {
    document.head.removeChild(styleTag)
  }
  this.gameContainer.innerHTML = mainDiv.innerHTML
  this.drawer.setScreen(this.gameContainer, engineConfig.resolution)
  // Only append style tag for regular scenes
  const styleElement = doc.head.getElementsByTagName('style')[0]
  if (styleElement) {
    document.head.appendChild(styleElement)
  }
} else {
  this.gameContainer.appendChild(mainDiv)
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Styleタグが増殖する</issue_title>
<issue_description>

---

Trelloから移行 (doing): https://trello.com/c/J1KHS9hw</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Styleタグが増殖する #118

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