fix(frontend): use system-ui for system font (#11177)

* fix(frontend): correct system font stack

This was originally set to Hiragino Maru Gothic Pro, which is the same as the current default font.

* just use system-ui

per code review https://github.com/misskey-dev/misskey/pull/11177#discussion_r1257260039



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Co-authored-by: Kagami Sascha Rosylight <saschanaz@outlook.com>