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)

locales/en/common.json

@@ -362,6 +362,8 @@
     "upload_progress": "Uploading {uploaded} / {total}",
     "upload_cancel": "Cancel upload",
     "upload_failed": "Failed to upload {filename}",
+    "drop_files": "Drop files to attach",
+    "show_less": "Show less",
     "send_failed": "Failed to send email",
     "continue_draft": "Continue draft",
     "close_draft_title": "Save or discard draft?",