feat: iframe-based email rendering with smart dark mode support

- Render HTML emails in sandboxed iframe (srcdoc) for true-to-life
  display with complete CSS isolation from app styles
- Detect emails with native dark mode (prefers-color-scheme) and
  let them handle their own theming
- Apply CSS filter inversion for dark mode on emails without native
  support, with re-inversion for images/media to preserve appearance
- Add per-email light/dark toggle button (Sun/Moon icon) next to
  email size, resets on email change (not persisted)
- Fix HTML reply/forward to include original email HTML content
- Send replies as multipart/alternative (text + HTML)
- Add drag-and-drop file attachments with overlay indicator
- Auto-resize composer textarea to avoid double scrolling
- Pin attachments section and bottom toolbar outside scroll area
- Collapsible attachment list (show 3, toggle for more)

app/[locale]/page.tsx

@@ -423,6 +423,7 @@ export default function Home() {
     bcc: string[];
     subject: string;
     body: string;
+    htmlBody?: string;
     draftId?: string;
     fromEmail?: string;
     fromName?: string;
@@ -431,7 +432,7 @@ export default function Home() {
     if (!client) return;
 
     try {
-      await sendEmail(client, data.to, data.subject, data.body, data.cc, data.bcc, data.identityId, data.fromEmail, data.draftId, data.fromName);
+      await sendEmail(client, data.to, data.subject, data.body, data.cc, data.bcc, data.identityId, data.fromEmail, data.draftId, data.fromName, data.htmlBody);
       setShowComposer(false);
 
       // Refresh the current mailbox to update the UI
@@ -1349,6 +1350,7 @@ export default function Home() {
                     cc: selectedEmail.cc,
                     subject: selectedEmail.subject,
                     body: selectedEmail.bodyValues?.[selectedEmail.textBody?.[0]?.partId || '']?.value || selectedEmail.preview || '',
+                    htmlBody: selectedEmail.bodyValues?.[selectedEmail.htmlBody?.[0]?.partId || '']?.value || undefined,
                     receivedAt: selectedEmail.receivedAt
                   } : undefined)}
                   initialDraftText={composerDraftText}