fix: split view / raw markdown editor can't scroll in fullscreen (#1088)

[cycsmail]

Fixes #1088.

In fullscreen editing the split-view and raw-markdown panes can't be scrolled, while IR and WYSIWYG work fine. The reason is that sv/raw render as a single <pre class="vditor-sv vditor-reset"> where the container and the editable surface are the same element, so the existing rule

.vditor.fullscreen-editor .vditor-sv .vditor-reset { overflow-y: auto; }

never matches it (it's a descendant selector, but here .vditor-reset sits on the same element as .vditor-sv). The element is left with overflow: hidden from the group above, so it clips instead of scrolling. IR/WYSIWYG nest a separate .vditor-reset inside their container, which is why they were unaffected.

The fix adds a compound selector targeting that combined element so it scrolls itself:

.vditor.fullscreen-editor .vditor-sv.vditor-reset {
  display: block;
  overflow-y: auto;
  overflow-x: hidden;
}

Higher specificity than the overflow: hidden group, so the sv/raw pane resolves to overflow-y: auto. Non-fullscreen editing and IR/WYSIWYG are untouched.

The repo has no CSS/DOM test harness, so I checked this with a small standalone cascade resolver that parses vditor.css and computes the effective overflow-y for the fullscreen pre.vditor-sv.vditor-reset: with the fix it resolves to auto, and after stripping the added rule it goes back to hidden, confirming the checker tracks the actual bug and not just my edit.