fix(NcAppContent): background blur may be missing after minification

[ShGKme]

☑️ Resolves

• Fix: Background blur missing on left sidebar nextcloud/spreed#17991
• -webkit-backdrop-filter doesn't work in Chromium
• -webkit-backdrop-filter should be added by build tools only if needed (according to the browserslist config), we don't need it in the source
• When both prefixed and unprefixed values are present, and prefixed is the second, CSS optimization with LightningCSS (used in Vite and RSPack) keeps the prefixed value only
  • Bug: backdrop-filter dropped when -webkit-backdrop-filter present, breaking styles parcel-bundler/lightningcss#1229

Screenshots

Before	After

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[ShGKme]

/backport to stable8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.60%. Comparing base (d73eeaf) to head (c192573).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8544   +/-   ##
=======================================
  Coverage   54.60%   54.60%           
=======================================
  Files         106      106           
  Lines        3443     3443           
  Branches     1005     1005           
=======================================
  Hits         1880     1880           
  Misses       1322     1322           
  Partials      241      241           

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[nickvergessen]

Confirmed in Firefox devtools