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/renderer/src/terminal/TerminalView.tsx

@@ -778,7 +778,11 @@ export const TerminalView: ForwardRefExoticComponent<TerminalViewProps & { ref?:
       allowTransparency: true,
       overviewRuler: { width: 9, showTopBorder: false, showBottomBorder: false },
       drawBoldTextInBrightColors: true,
-      minContrastRatio: 1,
+      // In image mode the xterm background is transparent (#00000000), which
+      // xterm's contrast cache treats as black; a raised contrast floor lifts
+      // dark foreground colors (light themes) enough to stay readable over
+      // the wallpaper. Without an image keep 1 (off) to honor theme colors.
+      minimumContrastRatio: tSettings.backgroundImage ? 4.5 : 1,
       automaticFontFallback: true,
       convertEol: false,
       windowsPty: { backend: 'conpty', buildNumber: 0 }
@@ -1113,6 +1117,7 @@ export const TerminalView: ForwardRefExoticComponent<TerminalViewProps & { ref?:
     // terminals opened after the change, which reads as "the setting is broken".
     term.options.scrollback = tSettings.scrollback
     term.options.theme = withChromeColors(getThemeById(tSettings.themeId, settings.customThemes).colors as ITheme, Boolean(tSettings.backgroundImage))
+    term.options.minimumContrastRatio = tSettings.backgroundImage ? 4.5 : 1
     scheduleFit()
   }, [tSettings, settings.customThemes, scheduleFit])
 

src/renderer/src/terminal/terminal.css

@@ -26,6 +26,14 @@
   background-color: transparent;
 }
 
+/* With a background image the dock must dim toward a fixed dark base instead of
+   the theme chrome color: on light themes --chrome-bg is near-white, so lowering
+   the image opacity would wash the wallpaper out to white (text unreadable)
+   rather than darkening it. */
+.terminal-view.has-bg-image .terminal-view-dock {
+  background: #0d1117;
+}
+
 /* Slim slate scrollbar for the xterm viewport — the native Windows one renders
    light track + arrow buttons that look broken on the dark surface. The thumb
    sits inset inside a wider grab lane (transparent border + padding-box clip)