fix: render only inner border in monthly view

[lja1018]

Please check if the PR fulfills these requirements

• It's the right issue type on the title
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
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Description

• render only the inner border in monthly view

• AS-IS

• TO-BE

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