feat: custom terminal background image + theme editor hardening

- settings: backgroundImage/backgroundImageOpacity (10..100, default 60)
- main: otimg:// protocol serves only the configured background file
  (path-allowlisted, 403 otherwise); dev http origin cannot load file:
- TerminalView: allowTransparency + transparent theme background while an
  image is set; .term-bg-image layer behind the xterm surface
- terminal.css: .has-bg-image keeps .xterm-viewport transparent — the old
  chrome-bg pin covered the image layer (root cause of image not showing)
- ThemeSettingsTab: image picker + opacity slider
- ThemeEditor: duplicate-name hint now covers builtin names too; seed
  colors normalized to #rrggbb
- settingsStore: sanitize theme colors, reject non-hex values

src/shared/i18n/dicts/zh-CN/settings.ts

@@ -133,12 +133,18 @@ const settings: Record<string, string> = {
   'settings.highlight.builtin.url': '网址链接',
   'settings.theme.accent': '标签强调色',
   'settings.theme.accentDesc': '活动标签边框、侧栏指示条与 SSH 徽章的强调颜色',
+  'settings.theme.backgroundImage': '背景图片',
+  'settings.theme.backgroundImageDesc': '为终端设置一张背景图片；移除后恢复纯色背景',
+  'settings.theme.chooseImage': '选择图片',
+  'settings.theme.clearImage': '移除图片',
+  'settings.theme.imageOpacity': '图片不透明度',
   'settings.theme.customBadge': '自定义',
   'settings.theme.deleteTitle': '删除自定义主题',
   'settings.theme.create': '新建主题',
   'settings.theme.editTitle': '编辑主题',
   'settings.themeEditor.name': '名称',
   'settings.themeEditor.namePlaceholder': '主题名称',
+  'settings.themeEditor.duplicateName': '与其他主题重名，仍可保存',
   'settings.themeEditor.copySuffix': '（副本）',
   'settings.about.currentVersion': '当前版本',
   'settings.about.statusIdle': '点击检查更新',