Remove gap between bottom of window and horizontal sidebar using flexbox

[wx-03]

Summary:

Fixes #416

Type of change:

• 🐛 Bug Fix

Changes Made:

• Initially, each column was set to a height of 77vh in card-view component, which resulted in a weird gap between the bottom of the window and the horizontal sidebar on screens that have enough vertical height.
• Setting the height to (100vh - filter bar height - header height) may not be a feasible solution as the heights will be hardcoded and need to be changed when the height of the filter bar / header changes.
• Thus I modified app-root to be a flexbox, which allows the content to take up the remaining vertical space of the viewport.

Screenshots:

Before:

After:

Proposed Commit Message:

Remove gap between bottom of window and horizontal sidebar

The height of each column in `card-view` component is set to 77vh,
which results in a gap between the bottom of the window and the
horizontal sidebar. 

The content should fill up the remaining height of the window instead
of leaving a gap.

Let's use a flexbox to allow the content to dynamically change its
height to fill up the remaining space instead of having a fixed height.

Checklist:

• I have tested my changes thoroughly.
• I have created tests for any new code files created in this PR or provided a link to a issue/PR that addresses this.
• I have added or modified code comments to improve code readability where necessary.
• I have updated the project's documentation as necessary.

[Eclipse-Dominator]

GUI works as expected. More space is now utilized!

LGTM!