fix(Files): remove stale addStyle icons call from TemplateLoader

[vitormattos]

Summary

Removes the dead Util::addStyle(Application::APP_ID, 'icons') call from TemplateLoader::handle().

Root cause

The css/icons.css file does not exist in the Vite build output. Its content (.icon-libresign { background-image: ... }) is already bundled inside libresign-tab.css because tab.ts imports ./style/icons.scss.

Having this stale addStyle call means:

1. Every time the Files sidebar is opened, Nextcloud queues a request for a non-existent CSS file → 404 on every page load.
2. If that file existed (or ever existed in a cache), any unscoped rule inside it would bleed globally into other Nextcloud apps — exactly the list-style: none regression reported in libresign css affect list-style all over nextcloud #7632.

Changes

• lib/Files/TemplateLoader.php: Remove the stale Util::addStyle(Application::APP_ID, 'icons') line.
• tests/php/Unit/Files/TemplateLoaderTest.php: Add regression test testHandleDoesNotRegisterIconsStyleSeparately() that asserts libresign/css/icons is never registered as a standalone stylesheet after handle() is called.

Ref #7632

[vitormattos]

/backport to stable33

[vitormattos]

/backport to stable32

[backportbot-libresign]

The backport to stable32 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable32
git pull origin stable32

# Create the new backport branch
git checkout -b backport/7641/stable32

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 3c7a8087

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/7641/stable32

Error: Failed to push branch backport/7641/stable32: remote: Invalid username or token. Password authentication is not supported for Git operations.
fatal: Authentication failed for 'https://github.com/LibreSign/libresign.git/'

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[vitormattos]

/backport to stable32