fix(browse): YAML editor cut off at viewport bottom

.preview-pane__body was flex: 1 + display: flex; flex-direction:
column but without min-height: 0. The flex item's default
min-height is min-content (its natural content size), so when the
YAML editor's CodeMirror viewport carried many lines, the body
grew to fit the editor instead of letting the editor scroll
internally. The chain ran out of viewport before reaching the
editor's bottom edge; the body's own scroll bottomed out at a
height that still cropped the last few lines.

Adding min-height: 0 lets the body shrink to its flex-allocated
size so CodeMirror's internal scroll takes over correctly. Same
root cause as the standard flex+overflow papercut documented in
half the CSS guides on the internet — fine to add unconditionally,
no other consumers of .preview-pane__body care.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

browse/css/tree.css

@@ -244,6 +244,11 @@ body {
 
 .preview-pane__body {
     flex: 1;
+    min-height: 0;          /* critical: lets the flex child shrink to fit
+                               the viewport instead of growing to its
+                               content's natural size (which clips the
+                               YAML editor's bottom when there are many
+                               lines, even with the editor's own scroll) */
     overflow: auto;
     display: flex;
     flex-direction: column;