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',