feat: custom terminal background image + theme editor hardening

- settings: backgroundImage/backgroundImageOpacity (10..100, default 60)
- main: otimg:// protocol serves only the configured background file
  (path-allowlisted, 403 otherwise); dev http origin cannot load file:
- TerminalView: allowTransparency + transparent theme background while an
  image is set; .term-bg-image layer behind the xterm surface
- terminal.css: .has-bg-image keeps .xterm-viewport transparent — the old
  chrome-bg pin covered the image layer (root cause of image not showing)
- ThemeSettingsTab: image picker + opacity slider
- ThemeEditor: duplicate-name hint now covers builtin names too; seed
  colors normalized to #rrggbb
- settingsStore: sanitize theme colors, reject non-hex values

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

@@ -139,12 +139,18 @@ const settings: Record<string, string> = {
   'settings.highlight.builtin.url': 'Web links',
   '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',
+  'settings.theme.chooseImage': 'Choose image',
+  'settings.theme.clearImage': 'Remove',
+  'settings.theme.imageOpacity': 'Image opacity',
   'settings.theme.customBadge': 'Custom',
   'settings.theme.deleteTitle': 'Delete custom theme',
   'settings.theme.create': 'New theme',
   'settings.theme.editTitle': 'Edit theme',
   'settings.themeEditor.name': 'Name',
   'settings.themeEditor.namePlaceholder': 'Theme name',
+  'settings.themeEditor.duplicateName': 'Same name as another theme (saving is still allowed)',
   'settings.themeEditor.copySuffix': ' (copy)',
   'settings.about.currentVersion': 'Current version',
   'settings.about.statusIdle': 'Click to check for updates',