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/en/settings.ts

@@ -284,7 +284,7 @@ const settings: Record<string, string> = {
   'settings.theme.accent': 'Tab accent color',
   'settings.theme.accentDesc': 'Accent color for the active tab border, the sidebar indicator, and SSH badges',
   'settings.theme.backgroundImage': 'Background image',
-  'settings.theme.backgroundImageDesc': 'Show an image behind the terminal; remove it to go back to a solid background. Lower the opacity if text gets hard to read (20–40% works well for bright images)',
+  'settings.theme.backgroundImageDesc': 'Show an image behind the terminal; remove it to go back to a solid background. In every theme the image dims toward dark and text is brightened automatically; lower the opacity if it is still hard to read (20–40% works well for bright images)',
   'settings.theme.chooseImage': 'Choose image',
   'settings.theme.clearImage': 'Remove',
   'settings.theme.imageOpacity': 'Image opacity',

src/shared/i18n/dicts/ja/settings.ts

@@ -279,7 +279,7 @@ const settings: Record<string, string> = {
   'settings.theme.accent': 'タブのアクセントカラー',
   'settings.theme.accentDesc': 'アクティブなタブの枠線、サイドバーのインジケーター、SSH バッジのアクセントカラー',
   'settings.theme.backgroundImage': '背景画像',
-  'settings.theme.backgroundImageDesc': 'ターミナルの背後に画像を表示します。削除すると無地の背景に戻ります。文字が読みにくい場合は不透明度を下げてください（明るい画像は 20–40% が目安です）',
+  'settings.theme.backgroundImageDesc': 'ターミナルの背後に画像を表示します。削除すると無地の背景に戻ります。どのテーマでも画像は暗い側へ減光され、文字は自動で明るくなります。それでも読みにくい場合は不透明度を下げてください（明るい画像は 20–40% が目安です）',
   'settings.theme.chooseImage': '画像を選択',
   'settings.theme.clearImage': '削除',
   'settings.theme.imageOpacity': '画像の不透明度',

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',

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

@@ -268,7 +268,7 @@ const settings: Record<string, string> = {
   'settings.theme.accent': '標籤強調色',
   'settings.theme.accentDesc': '作用中標籤的邊框、側邊欄指示條與 SSH 徽章的強調顏色',
   'settings.theme.backgroundImage': '背景圖片',
-  'settings.theme.backgroundImageDesc': '為終端設定一張背景圖片；移除後恢復純色背景；文字看不清時調低透明度（亮圖建議 20–40%）',
+  'settings.theme.backgroundImageDesc': '為終端設定一張背景圖片；移除後恢復純色背景；任何主題下圖片都向深色壓暗、文字自動提亮，仍看不清時調低透明度（亮圖建議 20–40%）',
   'settings.theme.chooseImage': '選擇圖片',
   'settings.theme.clearImage': '移除圖片',
   'settings.theme.imageOpacity': '圖片不透明度',