[vlidholt]

Summary

Follow-up to #36. Images exported from Write as Markdown with images came out as .png files, and some had a large transparent area around the picture.

• Cause: Write's client-side export re-encoded every image that wasn't PNG or JPEG into PNG with Flutter's Image.toByteData. Since feat(files): store uploaded images as WebP with Pixtronaut; drop BMP #36 every stored image is WebP, so every image took that path. On Flutter web (wasm), toByteData(format: png) resizes a shared offscreen canvas and then copies the whole canvas into the PNG, and those steps aren't atomic. When another image is rasterized in between, the PNG gets that canvas's size, with the image in its top-left corner. A reported file was 1627 × 2000 with the picture at 1:1 in the corner.
• Fix: decodeForExport describes PNG/JPEG/GIF/WebP from their header with describeExportImage, as the server-side exports already did, and passes them through unchanged. Markdown (images/<n>-<slug>.webp) and HTML exports keep the WebP. Only other formats (SVG, AVIF of https images) still go through Flutter.
• Office formats: DOCX/ODT/RTF/EPUB already convert in pure Dart (prepareOfficeImage). Opaque lossy WebP (photos) now becomes JPEG at quality 90 instead of a much larger PNG; lossless or transparent WebP becomes PNG. PDF decodes WebP itself, unchanged.
• Sheets: Download image names the file by its sniffed type instead of assuming PNG for bytes from the server.
• Blog submissions: checked, nothing to change. The webhook's assets[].type/name and the asset download's Content-Type come from the stored file (image/webp, *.webp), and submission_cover_test checks this. The webhook examples in docs/blog-integration.md and specs/18 now show a WebP asset.
• Write spec 07 §4.4 updated.

This fix was first pushed to main by mistake (595119b) and reverted there (3a62134). This PR brings it back.

Testing

• dart analyze and dart format clean.
• New: export_service_test (a WebP passes the client resolver unchanged and is named .webp) and office_images_test (lossy opaque WebP → JPEG; lossless or transparent WebP → PNG).
• wonky_write_core test/export (295), Write test/features/export and test/features/images, and the Sheets object/image command tests pass.
• Not reproduced in a browser: the padding was diagnosed from the reported file and Flutter's skwasm source.

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