feat(viewer): show a picture's blurhash, at its size, while it loads

[skjnldsv]

While a picture loads, the viewer shows a spinner on an empty backdrop. When the server keeps the picture's blurhash and its size, it now shows its blurred colours instead, in the box the picture will take, so nothing moves when it lands.

Relates to nextcloud/viewer#703

Loading	Loaded

A blurhash says nothing of a picture's proportions, so it takes both: metadata-blurhash, which the server generates, and metadata-photos-size, which the Photos app records with the EXIF rotation applied. Photos nodes carry both. Files asks for the blurhash only, so the image handler has Files lists ask for the size too. Registering that property pulls the webdav client into the registration chunk until @nextcloud/files ships nextcloud-libraries/nextcloud-files#1792.

The size also sets how far the picture zooms before the sharper preview has been fetched. It is only a starting point that loading can raise: Photos fills in 256×256 for a file it has not read yet.

Component tests cover the placeholder's size, the spinner alone with a blurhash but no size, and the zoom. A playground test opens photo.jpg with its real blurhash and size, slowed down, and checks the placeholder's proportions and that the picture lands in the same box.

👾 This pull request was assisted by Claude Code, commits carry an Assisted-by trailer.

[codecov]

Codecov Report

❌ Patch coverage is 98.18182% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.42%. Comparing base (6cb232b) to head (97a3cf6).

Files with missing lines	Patch %	Lines
lib/components/Images.vue	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   92.34%   92.42%   +0.07%     
==========================================
  Files          48       49       +1     
  Lines        4051     4103      +52     
  Branches      887      898      +11     
==========================================
+ Hits         3741     3792      +51     
- Misses        289      290       +1     
  Partials       21       21              

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[artonge]

I don't see a call to registerDavProperty for those two props, I assume the Viewer simply uses them if it finds them?

[skjnldsv]

I don't see a call to registerDavProperty for those two props, I assume the Viewer simply uses them if it finds them?

nope, it's absolutely needed, you're right!
We just need nextcloud-libraries/nextcloud-files#1792 too to ensure we don't ship the whole webdav code here too