Drag handle to set code block height with internal scrolling

Auto-collapse of tall code blocks (#11967, refined in #12743 and #13815) fixed the worst case of giant blocks dominating a document — long docs are much easier to scroll now. The height control is still binary though: a block is either collapsed to 10 lines or expanded to full height. For a 300-line block, expanding means the page grows enormous again, and there's no way for an author to say "this is reference material, a 15-line window with internal scrolling is enough". Proposal Give code blocks the same resize interaction images and videos already have: a drag handle at the bottom edge to set the visible height. • Dragging sets a fixed height with internal scrolling (max-height + overflow-y: auto), so the block stops growing the page • Bounded: minimum of a couple of lines, capped at the block's natural height • The height persists on the node, so all readers see the same layout • Double-click resets to automatic height (same as image…