feat: Add invite compressor and compressed header display

- Add InviteCompressor for terminal-friendly calendar invite summaries
- Add test fixtures for large group invite and cancellation emails
- Compress To/CC headers to single line with '... (+N more)' truncation
- Add 'h' keybinding to toggle between compressed and full headers
- EnvelopeHeader now shows first 2 recipients by default

src/mail/app.py

@@ -914,6 +914,17 @@ class EmailViewerApp(App):
         """Scroll the main content up by a page."""
         self.query_one("#main_content").scroll_page_up()
 
+    def action_toggle_header(self) -> None:
+        """Toggle between compressed and full envelope headers."""
+        content_container = self.query_one("#main_content", ContentContainer)
+        if hasattr(content_container, "header") and content_container.header:
+            content_container.header.toggle_full_headers()
+            # Provide visual feedback
+            if content_container.header.show_full_headers:
+                self.notify("Showing full headers", timeout=1)
+            else:
+                self.notify("Showing compressed headers", timeout=1)
+
     def action_toggle_main_content(self) -> None:
         """Toggle the visibility of the main content pane."""
         self.main_content_visible = not self.main_content_visible