[FE Feat] Refactor user/admin dashboard layout

[juancwu]

Description

The current layout setup is not intuitive to work with because there are many layers of abstraction with a combination of file/module locations not being clear with multiple nested folders.

The idea is to just create a user AppShell component that has three sub-parts:

• AppShell.Side which is where the side bar will be
• AppShell.Header which is the header part of the layout
• AppShell.Main which is the main body of the layout

Hopefully, with this change and refactor, each user/admin page can define how they should each look while keeping the same layout as designed in Figma.

Screenshots

Checklist

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• Is labelled properly - if this is a refactor, add the label!
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