fix(editor): preserve capture size in exported PNGs

[jondkinney]

Opening a saved screenshot from a 2× monitor through Omaroll's Annotate action or a file browser treated its native pixels as logical dimensions, making it appear twice its original size. Store the rendered capture's logical dimensions in the exported PNG and restore them when reopening the file or clipboard image. Native pixels remain intact, including crops, backdrops, expanded canvases, and scrolling captures.

Editable operation logs retain precedence over the PNG dimensions, and untagged images keep their existing pixel-sized presentation. The fast PNG encoder writes the small size field directly; other metadata continues through Qt's existing fallback.

Validation:

• make check passed, including scale round trips at 1×, 1.25×, 1.5×, and 2×, Save As and preview exports, invalid metadata, exact pixel preservation, and unchanged compressed image payloads.
• A 6K PNG encoded in about 170–174 ms both with and without the metadata; the field added 42 bytes.
• Tested locally through Omaroll. The reopened 2,340 × 2,222 PNG matches the original 1,170 × 1,111 logical selection on a 2× display, and actual-size viewing in imv was confirmed.