Fix contributor graphs mobile layout and responsiveness (#29597)
Also removed a unneeded and actually conflicting class name
`stats-table`.
Fixes: https://github.com/go-gitea/gitea/issues/29192
<img width="445" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/787804ed-6ba4-437f-b314-f23cbe2edf7a">
(cherry picked from commit f14779592494d41b3ab04caaab53487f2f4ede5a)
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