fix(html): name a linked ooxml image relative to the document

[andiwand]

🤖 Generated with Claude Code

Closes opendocument-app/OpenDocument.droid#551.

Every image in a .docx / .xlsx renders as the broken-image placeholder when the host links images rather than embedding them. ODF is fine, which is what made it easy to miss.

Cause

The engines disagree on what an image href is. odf_document.cpp:935 returns the raw xlink:href, which is relative — Pictures/x.png. The ooxml ones resolve the relationship into an absolute container path — ooxml_text_document.cpp:573 joins onto AbsPath("/word"), ooxml_spreadsheet_document.cpp:416 onto the part's directory — and translate_image emitted whichever it got.

An absolute location cannot work. resource_at matches it against the request path by exact string, and the http route strips the leading /, so /word/media/image1.jpeg served under /file/<prefix>/ misses either way. bring_offline is worse than a 404: it does Path(output).join(RelPath(*location)) and RelPath throws on an absolute string, so translate --embed-images=false on a docx fails before writing anything.

Neither knob the downstream issue proposed helps. relative_resource_paths gates its rebase on resource.is_shipped(), and HttpServer::prefix_pattern is ([a-zA-Z0-9_-]+), so there is no root to mount at.

Fix

Make the location relative where the resource is built, once, for every engine:

const std::string path = Path(image.href()).make_relative().string();

image_file() and image_is_internal() already make_absolute() the href, so the engines stay as they are and Image::href() keeps its meaning.

Tests

html.linked_images_are_served renders an odt, a docx and an xlsx with embed_images = false and asserts each image location is relative, exists on the service, and serves bytes. It fails on both ooxml paths without the change.

The reference output renders with embed_images = true, so it never exercised the linked form — that is the gap this closes. Its docx / xlsx / odt output is byte-identical after the change, since an embedded image carries no location.

Not in scope

Two neighbours found while confirming this, each worth its own issue:

• ooxml_text_parser.cpp:226 maps every a:graphicData to ElementType::image, so a chart, SmartArt or embedded OLE becomes an image with an empty href — the src="" seen in a handful of reference documents. file-sample_100kB.docx is one: a picture and a chart, and the chart is the empty one.
• document_element.cpp:508 creates a non-internal image resource with is_external = false.