fix: Header display, mode toggle, and help screen improvements

- Fix toggle_mode to pass envelope context when reloading (preserves compression)
- Add CSS for header labels: single-line with text-overflow ellipsis
- Add 'full-headers' CSS class for toggling between compressed/full view
- Store full subject in header for proper refresh on toggle
- Update HelpScreen with missing shortcuts (o, b) and better descriptions
- Fix duplicate line in _refresh_display

src/mail/email_viewer.tcss

@@ -73,10 +73,25 @@ StatusTitle {
 EnvelopeHeader {
   dock: top;
   width: 100%;
-  max-height: 2;
+  height: auto;
+  max-height: 10;
+  padding: 0 1;
   tint: $primary 10%;
 }
 
+/* Header labels should be single line with truncation */
+EnvelopeHeader Label {
+  width: 100%;
+  height: 1;
+  text-overflow: ellipsis;
+}
+
+/* Full headers mode - allow wrapping */
+EnvelopeHeader.full-headers Label {
+  height: auto;
+  text-overflow: clip;
+}
+
 Markdown {
   padding: 1 2;
 }