feat: add loading screen to admin

[gweiermann]

Closes https://github.com/shopware/shopware/issues/13741

What has been done?

• loading indicator gets visible after 2s (instead of 3s), which was suggested by @fabianhueske and also feels more responsive. Can be changed back to 3s if needed
• If the indicator is visible, it stays visible for at least 300ms. This may increases loading time by 300ms worst case but prevents flickering of the loading indicator which leads to a better experience.
• The indicator is a copy of mt-loader with small adjustments

My only concern is that index.html feels a bit bloated.

https://github.com/user-attachments/assets/c34c8859-6dfd-418b-b1a2-79e89cda05af

[fabianhueske]

Hi @gweiermann ✌️ Good work. Is the jump in size/position at seconds 6-7 in your video somehow avoidable?

[explore-openapi[bot]]

OpenAPI Snapshot

Project: shopware/store-api

No changes detected

Your OpenAPI schema is identical to the base branch (trunk).

Links:
All Project Snapshots · Base Branch Snapshot

Project: shopware/admin-api

No changes detected

Your OpenAPI schema is identical to the base branch (trunk).

Links:
All Project Snapshots · Base Branch Snapshot

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 54.14%. Comparing base (8368dbc) to head (98b90f6). :warning: Report is 3 commits behind head on trunk. :white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk   #13902      +/-   ##
==========================================
- Coverage   54.14%   54.14%   -0.01%     
==========================================
  Files        3204     3204              
  Lines       96425    96421       -4     
==========================================
- Hits        52210    52206       -4     
  Misses      44215    44215              

Flag	Coverage Δ
phpunit-migration	92.92% <ø> (ø)
phpunit-unit	51.85% <ø> (-0.01%)	:arrow_down:

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[gweiermann]

Hi @gweiermann ✌️ Good work. Is the jump in size/position at seconds 6-7 in your video somehow avoidable?

Yes I fixed it, good catch!

[BrocksiNet]

Same here:

The Pull Request doesn't contain any release info, if your changes are relevant for external developers please add an entry to the release info file, including the consequences of the change and how it affects external developers. For detailed infos please refer to the release documentation guide.

Just a small sentence would be great 👏🏼