feat(sidebar): add API for mounting the sidebar

[susnux]

• for feat(files): bring back the Sidebar API nextcloud/server#63488

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.01%. Comparing base (b6b0a2f) to head (e3988de).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1728      +/-   ##
==========================================
+ Coverage   85.94%   87.01%   +1.06%     
==========================================
  Files          44       44              
  Lines        1103     1109       +6     
  Branches      277      278       +1     
==========================================
+ Hits          948      965      +17     
+ Misses        117      106      -11     
  Partials       38       38              

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