fix(navigation): scope all styles to the calendar app

[st3iny]

Fix nextcloud/server#44638

Once again, our global app navigation styles leak to other apps. Scoping them should be a safe operation as the navigation sidebar on the left is not shown outside of the app.

Best reviewed as: https://github.com/nextcloud/calendar/pull/6186/files?w=1

Files is now fixed

Before	After

No visual regressions

There are no visual regressions due to changed style precedence.

Before	After

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 23.97%. Comparing base (ffccb86) to head (307a265).
Report is 92 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6186   +/-   ##
=========================================
  Coverage     23.97%   23.97%           
  Complexity      457      457           
=========================================
  Files           247      247           
  Lines         11606    11606           
  Branches       2178     2188   +10     
=========================================
  Hits           2782     2782           
+ Misses         8509     8508    -1     
- Partials        315      316    +1     

Flag	Coverage Δ
javascript	15.46% <ø> (ø)
php	59.53% <ø> (ø)

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[ChristophWurst]

https://github.com/nextcloud/calendar/pull/6186/files?w=1 👍

[st3iny]

/backport to stable4.7

Ref nextcloud/mail#9933