fix(docx-export): embed data-URI images instead of leaking base64 text

[harsh-thakkar7]

_resolve_image_source handled file://, http(s), and relative paths, but not data: URIs. An inline image like ![pixel](data:image/png;base64,iVBORw0K...) fell through to the on-disk path candidates, returned (src, None), and the <img> handler wrote [data:image/png;base64,iVBORw0K...] as the paragraph's literal text — a raw base64 blob visible in the exported DOCX, with no embedded picture.

Fix: decode data: URIs in _resolve_image_source (base64 for ;base64 payloads, URL-unquote otherwise) and return the bytes so add_picture() embeds the image.

Adds test_data_uri_image_is_embedded_not_leaked_as_text, which failed on main (the blob appeared as paragraph text and inline_shapes was empty) and passes here (no data: text, one inline shape).

[petertzy]

Approved — this fixes the DOCX export path for inline data-URI images by decoding the payload and passing image bytes to add_picture(), so Base64 content is embedded as an image instead of leaking into document text.
I also added compatibility coverage for case-insensitive data-URI metadata and URL-encoded image payloads. Backend tests (474 passed), frontend tests (94/94), Ruff, ESLint, and TypeScript checks all pass.