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/renderer/index.html

@@ -5,7 +5,7 @@
     <title>OpenTerminal</title>
     <meta
       http-equiv="Content-Security-Policy"
-      content="default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; font-src 'self' data:"
+      content="default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; font-src 'self' data:; img-src 'self' file: data: otimg:"
     />
   </head>
   <body>