fix: add copy-to-clipboard feedback and neutralize dark mode background tint

Copy source button now shows a checkmark and "Copied" text for 2s
after clicking. Dark mode email background blend target changed to
perfectly neutral gray (R=G=B) to eliminate perceived color cast on
certain displays.

components/email/email-viewer.tsx

@@ -425,13 +425,15 @@ export function EmailViewer({
     return source;
   };
 
+  const [sourceCopied, setSourceCopied] = useState(false);
   const copySourceToClipboard = async () => {
     if (!email) return;
 
     try {
       const source = generateEmailSource(email);
       await navigator.clipboard.writeText(source);
-      // Could add a toast notification here
+      setSourceCopied(true);
+      setTimeout(() => setSourceCopied(false), 2000);
     } catch {
       return;
     }
@@ -1753,8 +1755,8 @@ export function EmailViewer({
                   onClick={copySourceToClipboard}
                   className="flex items-center gap-1.5"
                 >
-                  <Copy className="w-4 h-4" />
-                  {t('copy_source')}
+                  {sourceCopied ? <Check className="w-4 h-4 text-green-600 dark:text-green-400" /> : <Copy className="w-4 h-4" />}
+                  {sourceCopied ? tCommon('copied') : t('copy_source')}
                 </Button>
                 <Button
                   variant="ghost"

lib/color-transform.ts

@@ -155,7 +155,7 @@ export function transformBgColorForDarkMode(colorString: string): string {
   if (luminance < 0.2) return colorString;
 
   const blendFactor = Math.min(0.9, (luminance - 0.2) * 1.125);
-  const darkR = 30, darkG = 31, darkB = 38;
+  const darkR = 35, darkG = 35, darkB = 35;
 
   const r = Math.max(0, Math.round(rgb.r + (darkR - rgb.r) * blendFactor));
   const g = Math.max(0, Math.round(rgb.g + (darkG - rgb.g) * blendFactor));
@@ -244,7 +244,7 @@ export function generateIframeStylesheet(): string {
       @media (prefers-color-scheme: dark) {
         body {
           color: #e2e8f0;
-          background: #1e1f26;
+          background: #232323;
         }
         a { color: #60a5fa; }
         img { opacity: 0.9; }