feat(config, theme): add clientOnly marker

[zhangyx1998]

Description

• Provided clientOnly(object: any) and isClientOnly(object: any): boolean APIs to mark an object as client-only.
• Modified component ClientOnly.ts to accept an optional prop isClientOnly: Boolean so its behavior can be programmatically controlled.
• Updated navbar and sidebar components to allow optional client-only rendering.

Linked Issues

closes #3367

Additional Context

Before (22 occurrences of a sidebar item)

After (4 irrelevant occurrences)

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[zhangyx1998]

@brc-dd Seems like the data loading test on windows-latest could randomly timeout due to fluctuations of GitHub server. There is literally nothing changed in the previous failed test.

[BAGOMBEKA-JOB-DEV]

@zhangyx1998 You need to first resolve some of the conflicts in your code. Just try to cross check.

[zhangyx1998]

Thanks for reminding! I'll find time to resolve it asap.

[BAGOMBEKA-JOB-DEV]

Conflicts resolved!

[zhangyx1998]

Test/node20 failed because it lacks support for Iterator helpers.

The newly introduced package relies on Iterator.map and Iterator.filter. I am looking for workarounds without too much performance penalty.