feat: Add comprehensive help screen modal

- Create HelpScreen with all keyboard shortcuts
- Add hardcoded sections for instructions
- Add binding for '?' key to show help
- Support ESC/q/? to close help screen
- Document notification compression feature in help
- Format help with colors and sections (Navigation, Actions, View, Search)

Features:
- Shows all keyboard shortcuts in organized sections
- Quick Actions section explains notification compression
- Configuration instructions for mail.toml
- Modal dialog with close button
- Extracts bindings automatically for display

src/mail/screens/__init__.py

@@ -1,10 +1,11 @@
-# Initialize the screens package
+# Initialize screens package
 from .CreateTask import CreateTaskScreen
 from .OpenMessage import OpenMessageScreen
 from .DocumentViewer import DocumentViewerScreen
 from .LinkPanel import LinkPanel, LinkItem, extract_links_from_content
 from .ConfirmDialog import ConfirmDialog
 from .SearchPanel import SearchPanel, SearchHelpModal
+from .HelpScreen import HelpScreen
 
 __all__ = [
     "CreateTaskScreen",
@@ -16,4 +17,5 @@ __all__ = [
     "ConfirmDialog",
     "SearchPanel",
     "SearchHelpModal",
+    "HelpScreen",
 ]