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/globals.css

@@ -242,6 +242,26 @@ body {
   list-style-type: decimal;
 }
 
+/* Reply quoted HTML - preserves original inline styles/colors */
+.email-reply-quote {
+  overflow-wrap: break-word;
+  word-wrap: break-word;
+  max-width: none;
+}
+
+.email-reply-quote p {
+  margin: 0.5rem 0;
+}
+
+.email-reply-quote img {
+  max-width: 100%;
+  height: auto;
+}
+
+.email-reply-quote a {
+  text-decoration: underline;
+}
+
 /* Only style tables that are actual data tables, not layout tables */
 .email-content table.data-table,
 .email-content table[border="1"] {