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/email_viewer.tcss

@@ -71,12 +71,23 @@ StatusTitle {
 }
 
 EnvelopeHeader {
-  dock: top;
   width: 100%;
   height: auto;
-  max-height: 10;
+  min-height: 4;
+  max-height: 6;
   padding: 0 1;
-  tint: $primary 10%;
+  background: $surface-darken-1;
+  scrollbar-size: 1 1;
+}
+
+/* Full headers mode - allow more height for scrolling */
+EnvelopeHeader.full-headers {
+  max-height: 12;
+}
+
+#content_scroll {
+  height: 1fr;
+  width: 100%;
 }
 
 /* Header labels should be single line with truncation */