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/app.py

@@ -5,6 +5,7 @@ from .widgets.EnvelopeListItem import EnvelopeListItem, GroupHeader
 from .screens.LinkPanel import LinkPanel
 from .screens.ConfirmDialog import ConfirmDialog
 from .screens.SearchPanel import SearchPanel
+from src.mail.screens.HelpScreen import HelpScreen
 from .actions.task import action_create_task
 from .actions.open import action_open
 from .actions.delete import delete_current
@@ -142,6 +143,7 @@ class EmailViewerApp(App):
             Binding("A", "accept_invite", "Accept invite"),
             Binding("D", "decline_invite", "Decline invite"),
             Binding("T", "tentative_invite", "Tentative"),
+            Binding("?", "show_help", "Show Help"),
         ]
     )
 
@@ -652,10 +654,15 @@ class EmailViewerApp(App):
             self.action_newest()
 
     async def action_toggle_mode(self) -> None:
-        """Toggle the content mode between plaintext and markdown."""
+        """Toggle of content mode between plaintext and markdown."""
         content_container = self.query_one(ContentContainer)
         await content_container.action_toggle_mode()
 
+    async def action_show_help(self) -> None:
+        """Show help screen with keyboard shortcuts."""
+        help_screen = HelpScreen()
+        self.push_screen(help_screen)
+
     def action_next(self) -> None:
         if not self.current_message_index >= 0:
             return