feat(ImageView): use Photos metadata for pre-sized, blurhashed image placeholders

[emberfiend]

📝 Summary

• Resolves: Image loading moves text #4242 @provokateurin
• Resolution to photos/#2243 not included

This introduces a cross-app "dependency" on metadata currently provided by image uploads when Photos is enabled (although it fails gracefully without it). When present in oc_files_metadata, this uses photos-size.value.width and height (and blurhash.value) to show a placeholder of correct size before the image itself loads.

Notes:

1. While it's slick on Firefox, Chrome (or Chromium anyway) has a noticeable gap between removing the placeholder and adding the loaded image. Any ideas? (I'd like to fix this before merge ideally.) Fixed, the preload wasn't working because of some cache invalidation, so it had to download the image twice. Thanks Julius :)
2. This adds some brittle-ness with the hardcoded values in the CSS and size calculations. Now whenever we change the ImageView we need to make sure the placeholder is the same size. Open to ideas about ways to mitigate this.
3. Many ResizeObservers running on a long document mean that this might make window resizing laggy on slower devices. You could make the case that we just size placeholders once, for the initial load's element width, or pass one in from the parent (Editor.vue already watches width).
4. The blurhash logic could be abstracted out since it's similar to what's in photos, but I feel one repetition does not justify it yet. As always, happy to accommodate if anyone feels differently. Now uses NcBlurHash.
5. Julius pointed out that this does nothing for resources AttachmentResolver doesn't provide metadata for, such as DAV-sourced bare URLs. Something to add in the future.
6. On the todo front, I started this PR moving SizeMetadataProvider out of Photos to server, but after discussing with Julius we decided to split the task and do that separately & in conversation with the involved parties. Just flagging it as a follow-up here.

General pointers about Vue best practices or local conventions also appreciated.

🖼️ Videos

🏚️ Before
resize before.webm

🏡 After (firefox)
resize after.webm

🏡 After (chromium)
resize after chrome.webm
resize after chrome.webm

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits
• [😇] Tests (unit, integration and/or end-to-end) passing and the changes are covered with tests
• [N/A] Documentation (README or documentation) has been updated or is not required

[codecov]

Codecov Report

Attention: Patch coverage is 4.83871% with 59 lines in your changes missing coverage. Please review.

Project coverage is 59.00%. Comparing base (72e3453) to head (c1de72b).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nodes/ImageView.vue	4.83%	59 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7275      +/-   ##
==========================================
+ Coverage   52.18%   59.00%   +6.81%     
==========================================
  Files         483      482       -1     
  Lines       41918    37081    -4837     
  Branches     1047     1046       -1     
==========================================
+ Hits        21875    21878       +3     
+ Misses      19940    15101    -4839     
+ Partials      103      102       -1     

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[juliusknorr]

Approving already so it can get in after the @nextcloud/vue bump is in

[juliusknorr]

As discussed not a blocker but would be nice to base this and the paddings on the image view on the css variables so they adjust if designers change their minds about spacing.