feat(theme): add background image dim scrim slider

New terminal.backgroundImageDim setting (0-90%, default 0 = off). A black
scrim layer sits between the background image and the xterm screen, so
bright wallpapers stay readable at any opacity: unlike the opacity slider
(which blends the image toward the dark dock base), the scrim darkens the
image while preserving its saturation, and the raised minimumContrastRatio
(4.5) keeps lifted text legible on top.

- settings: backgroundImageDim with deepMerge type-fallback sanitize
- TerminalView: render .term-bg-dim only when image set and dim > 0
- ThemeSettingsTab: slider follows the existing draft + onChangeComplete
  pattern (no settings writes while dragging)
- i18n: settings.theme.backgroundImageDim(+Desc) in zh-CN/zh-TW/en/ja
- AGENTS.md: document the third image-mode invariant

src/renderer/src/settings/ThemeSettingsTab.tsx

@@ -32,6 +32,7 @@ export function ThemeSettingsTab({
   const setCustomThemes = useSettingsStore((s) => s.setCustomThemes)
   const activeTheme = useResolvedTheme()
   const [opacityDraft, setOpacityDraft] = useState<number | null>(null)
+  const [dimDraft, setDimDraft] = useState<number | null>(null)
 
   const themeCards = useMemo(
     () => [...BUILTIN_THEMES, ...settings.customThemes],
@@ -90,6 +91,26 @@ export function ThemeSettingsTab({
           />
         </div>
       )}
+      {settings.terminal.backgroundImage !== '' && (
+        <div className="theme-accent-row">
+          <div>
+            <div className="settings-block-label">{t('settings.theme.backgroundImageDim')}</div>
+            <div className="settings-block-hint">{t('settings.theme.backgroundImageDimDesc')}</div>
+          </div>
+          <Slider
+            style={{ width: 200 }}
+            min={0}
+            max={90}
+            step={5}
+            value={dimDraft ?? settings.terminal.backgroundImageDim}
+            onChange={(v) => setDimDraft(v)}
+            onChangeComplete={(v) => {
+              setDimDraft(null)
+              void updateTerminal({ backgroundImageDim: v })
+            }}
+          />
+        </div>
+      )}
       <div className="theme-accent-row">
         <div>
           <div className="settings-block-label">{t('settings.theme.accent')}</div>

src/renderer/src/terminal/TerminalView.tsx

@@ -1400,6 +1400,14 @@ export const TerminalView: ForwardRefExoticComponent<TerminalViewProps & { ref?:
               }}
             />
           )}
+          {settings.terminal.backgroundImage !== '' && settings.terminal.backgroundImageDim > 0 && (
+            <div
+              className="term-bg-dim"
+              style={{
+                opacity: Math.min(90, Math.max(0, settings.terminal.backgroundImageDim)) / 100
+              }}
+            />
+          )}
           {dead && (
             <div className="term-dead-mask">
               <div>{t('terminal.dead.message', { code: exitCode })}</div>

src/renderer/src/terminal/terminal.css

@@ -380,6 +380,17 @@
   pointer-events: none;
 }
 
+/* Black scrim between the background image and the xterm screen: dims the
+   image without washing it out (unlike opacity, which blends toward the dark
+   dock base), so bright images stay readable at any opacity. */
+.term-bg-dim {
+  position: absolute;
+  inset: 0;
+  z-index: 0;
+  background: #000;
+  pointer-events: none;
+}
+
 .terminal-view-dock > .xterm {
   position: relative;
   z-index: 1;