fix: scope global sidebar styles to the app

[st3iny]

How to reproduce?

1. Install Talk.
2. Open a chat room and open the right sidebar.
3. Observe the header of the right sidebar.

[st3iny]

/backport to stable4.7

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 29.29%. Comparing base (c01de6e) to head (2460c53). Report is 11 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6081 +/- ## ========================================= Coverage 29.29% 29.29% Complexity 914 914 ========================================= Files 287 287 Lines 13987 13987 Branches 2139 2150 +11 ========================================= Hits 4098 4098 + Misses 9575 9574 -1 - Partials 314 315 +1 ``` | [Flag](https://app.codecov.io/gh/nextcloud/calendar/pull/6081/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | Coverage Δ | | |---|---|---| | [javascript](https://app.codecov.io/gh/nextcloud/calendar/pull/6081/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `15.18% <ø> (ø)` | | | [php](https://app.codecov.io/gh/nextcloud/calendar/pull/6081/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `58.65% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud#carryforward-flags-in-the-pull-request-comment) to find out more.

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