fix(security): prevent global CSS injection from email style tags

Emails containing <style> tags were injecting global CSS rules that
affected the entire application UI. For example, button:hover rules
from emails would override all button hover states app-wide.

Changes:
- Block <style> tags in email sanitization config (security fix)
- Keep inline style attributes for element-specific formatting
- Add explicit bg-transparent to banner buttons (defense in depth)
- Add CSS isolation to banner container
- Fix theme dependency in email content memoization

This prevents malicious or poorly-formatted emails from breaking the
UI or being used for phishing via CSS injection attacks.

components/email/email-viewer.tsx

@@ -524,7 +524,7 @@ export function EmailViewer({
       html: '<p style="color: var(--color-muted-foreground);">No content available</p>',
       isHtml: false
     };
-  }, [email, allowExternalContent, hasBlockedContent, externalContentPolicy, isSenderTrusted]);
+  }, [email, allowExternalContent, hasBlockedContent, externalContentPolicy, isSenderTrusted, theme]);
 
   // Detect List-Unsubscribe header for newsletter banners
   const listHeaders = useMemo(() => {
@@ -1290,9 +1290,9 @@ export function EmailViewer({
         {/* Unified Notification Banner - External Content + Unsubscribe */}
         {((hasBlockedContent && !allowExternalContent && externalContentPolicy !== 'allow') ||
           (shouldShowUnsubBanner && listHeaders?.listUnsubscribe)) && (
-          <div className="border-b border-border bg-muted/30">
+          <div className="border-b border-border bg-muted/30 isolate">
             <div className="max-w-4xl mx-auto px-6 py-1.5">
-              <div className="flex flex-col md:flex-row md:items-center md:justify-center gap-3">
+              <div className="flex flex-col md:flex-row md:items-center md:justify-center gap-3 isolate">
                 {/* External Content Controls */}
                 {hasBlockedContent && !allowExternalContent && externalContentPolicy !== 'allow' && (
                   <div className="flex items-center gap-3 flex-wrap">
@@ -1300,7 +1300,7 @@ export function EmailViewer({
                     {externalContentPolicy === 'ask' && (
                       <button
                         onClick={() => setAllowExternalContent(true)}
-                        className="flex items-center gap-1.5 text-sm text-muted-foreground hover:text-foreground transition-colors min-h-[44px] md:min-h-0"
+                        className="flex items-center gap-1.5 text-sm text-muted-foreground hover:text-foreground bg-transparent hover:bg-transparent transition-colors min-h-[44px] md:min-h-0"
                       >
                         <Image className="w-3.5 h-3.5" />
                         {t('load_external_content')}
@@ -1316,7 +1316,7 @@ export function EmailViewer({
                             setAllowExternalContent(true);
                           }
                         }}
-                        className="flex items-center gap-1.5 text-sm text-muted-foreground hover:text-foreground transition-colors min-h-[44px] md:min-h-0"
+                        className="flex items-center gap-1.5 text-sm text-muted-foreground hover:text-foreground bg-transparent hover:bg-transparent transition-colors min-h-[44px] md:min-h-0"
                       >
                         {t('trust_sender')}
                       </button>

components/email/unsubscribe-banner.tsx

@@ -92,7 +92,7 @@ export function UnsubscribeBanner({
         </span>
         <button
           onClick={onDismiss}
-          className="text-sm text-muted-foreground hover:text-foreground transition-colors min-h-[44px] md:min-h-0"
+          className="text-sm text-muted-foreground hover:text-foreground bg-transparent hover:bg-transparent transition-colors min-h-[44px] md:min-h-0"
         >
           {t('email_viewer.unsubscribe_banner.dismiss')}
         </button>
@@ -110,14 +110,14 @@ export function UnsubscribeBanner({
           <button
             onClick={handleUnsubscribe}
             disabled={processing}
-            className="flex items-center gap-1.5 text-sm text-muted-foreground hover:text-foreground transition-colors disabled:opacity-50 disabled:cursor-not-allowed min-h-[44px] md:min-h-0"
+            className="flex items-center gap-1.5 text-sm text-muted-foreground hover:text-foreground bg-transparent hover:bg-transparent transition-colors disabled:opacity-50 disabled:cursor-not-allowed min-h-[44px] md:min-h-0"
           >
             {processing && <Loader2 className="w-3.5 h-3.5 animate-spin" />}
             {t('email_viewer.unsubscribe_banner.confirm_button')}
           </button>
           <button
             onClick={() => setShowConfirm(false)}
-            className="text-sm text-muted-foreground hover:text-foreground transition-colors min-h-[44px] md:min-h-0"
+            className="text-sm text-muted-foreground hover:text-foreground bg-transparent hover:bg-transparent transition-colors min-h-[44px] md:min-h-0"
           >
             {t('email_viewer.unsubscribe_banner.cancel')}
           </button>
@@ -125,7 +125,7 @@ export function UnsubscribeBanner({
       ) : (
         <button
           onClick={() => setShowConfirm(true)}
-          className="text-sm text-muted-foreground hover:text-foreground transition-colors min-h-[44px] md:min-h-0"
+          className="text-sm text-muted-foreground hover:text-foreground bg-transparent hover:bg-transparent transition-colors min-h-[44px] md:min-h-0"
         >
           {t('email_viewer.unsubscribe_banner.button')}
         </button>

lib/email-sanitization.ts

@@ -3,16 +3,18 @@ import DOMPurify from 'dompurify';
 /**
  * Unified DOMPurify configuration for email content
  * Blocks all script execution vectors while preserving formatting
+ * NOTE: <style> tags are forbidden to prevent global CSS injection
+ * Inline style attributes are still allowed for element-specific styling
  */
 export const EMAIL_SANITIZE_CONFIG = {
-  ADD_TAGS: ['style'],
+  ADD_TAGS: [],
   ADD_ATTR: ['target', 'style', 'class', 'width', 'height', 'align', 'valign', 'bgcolor', 'color'],
   ALLOW_DATA_ATTR: false,
   FORCE_BODY: true,
   FORBID_TAGS: [
     'script', 'iframe', 'object', 'embed', 'form',
     'input', 'button', 'meta', 'link', 'base',
-    'svg', 'math'
+    'svg', 'math', 'style'
   ],
   FORBID_ATTR: [
     'onerror', 'onload', 'onclick', 'onmouseover',