- MailListView: new '🌐 Online' button in search bar
- Clicking button emits onlineSearchRequested(query)
- If search box empty, shows QInputDialog for query entry
- MainMainWindow::onOnlineSearchRequested() handler:
- Validates current folder selected
- Gets account for folder
- Shows status message
- Shows info dialog with query/folder/account (placeholder for IMAP SEARCH)
- Signal: onlineSearchRequested(const QString& query)
- Button styled in blue (online theme)
- CompactMailDelegate: 3 display modes (Compact/Medium/Spacious) like WinUI original
* Compact: 64px, minimal, no preview
* Medium: 96px, with preview, detailed categories
* Spacious: 120px, full padding, preview visible
- Hover actions: Reply (↩), Forward (↪), Mark unread (✎) on right side
- Unread dot indicator (top-right, blue) like WinUI
- User-configurable display mode via combo box in filter bar
- MainMainWindow handlers for reply/forward/mark-unread from compact view
- Signals: replyRequested, forwardRequested, markUnreadRequested