fix(theme): dim background image toward dark and lift text contrast in image mode

On light themes the pane base is near-white (--chrome-bg), so lowering the
background image opacity washed the wallpaper out to white instead of
darkening it, and the theme's dark foreground text became unreadable.

- terminal.css: in has-bg-image mode the dock gets a fixed dark base
  (#0d1117), so the opacity slider always dims toward dark regardless of
  theme
- TerminalView: raise xterm minimumContrastRatio from 1 to 4.5 (WCAG AA,
  same as VS Code's terminal default) while an image is set; xterm treats
  the transparent background as black luminance, so dark foreground colors
  are lifted to stay readable over the wallpaper. Also fix the option name
  (minContrastRatio is silently ignored; the real key is
  minimumContrastRatio)
- i18n: update backgroundImageDesc in zh-CN/zh-TW/en/ja
- AGENTS.md: document the two image-mode invariants

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

@@ -268,7 +268,7 @@ const settings: Record<string, string> = {
   '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；文字看不清时调低透明度（亮图建议 20–40%）',
+  'settings.theme.backgroundImageDesc': '为终端设置一张背景图片；移除后恢复纯色背景；任何主题下图片都向深色压暗、文字自动提亮，仍看不清时调低透明度（亮图建议 20–40%）',
   'settings.theme.chooseImage': '选择图片',
   'settings.theme.clearImage': '移除图片',
   'settings.theme.imageOpacity': '图片不透明度',