feat: Scrollable envelope header with proper To/CC display

- Refactor ContentContainer to Vertical layout with fixed header + scrollable content
- Change EnvelopeHeader to ScrollableContainer for long recipient lists
- Parse headers from message content (fixes empty To: field from himalaya)
- Strip all email headers, MIME boundaries, base64 blocks from body display
- Add 22 unit tests for header parsing and content stripping
- Cancelled meeting emails now render with empty body as expected

src/mail/app.py

@@ -901,19 +901,19 @@ class EmailViewerApp(App):
 
     def action_scroll_down(self) -> None:
         """Scroll the main content down."""
-        self.query_one("#main_content").scroll_down()
+        self.query_one("#content_scroll").scroll_down()
 
     def action_scroll_up(self) -> None:
         """Scroll the main content up."""
-        self.query_one("#main_content").scroll_up()
+        self.query_one("#content_scroll").scroll_up()
 
     def action_scroll_page_down(self) -> None:
         """Scroll the main content down by a page."""
-        self.query_one("#main_content").scroll_page_down()
+        self.query_one("#content_scroll").scroll_page_down()
 
     def action_scroll_page_up(self) -> None:
         """Scroll the main content up by a page."""
-        self.query_one("#main_content").scroll_page_up()
+        self.query_one("#content_scroll").scroll_page_up()
 
     def action_toggle_header(self) -> None:
         """Toggle between compressed and full envelope headers."""