feat(input): add four-finger workspace overview gesture

[abhighansala]

Add four-finger swipe up to open the workspace overview and swipe down to close it, using the existing WorkspaceOverview system action.

What does this change do?

Adds four-finger vertical swipe gestures for the existing workspace overview:

• Four-finger swipe up opens the workspace overview.
• Four-finger swipe down closes the workspace overview.
• Existing four-finger horizontal workspace navigation is unchanged.

Why?

COSMIC already provides a workspace overview and supports four-finger touchpad gestures for workspace navigation. This adds the corresponding vertical gesture for opening and closing the existing overview.

Implementation

The gesture is handled in cosmic-comp and uses the existing System::WorkspaceOverview action, so the compositor does not duplicate any workspace overview UI logic.

Testing

Tested locally with:

• cargo fmt --check

• cargo check

• cargo build --release

• git diff --check

• Four-finger swipe up/down on the running COSMIC desktop

• Existing four-finger horizontal workspace gestures

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[git-f0x]

Duplicate of #1799. Adding more gestures is held up by #396.