114 add skeleton loading for pc components

[krgjert]

Purpose

Improve loading feedback with skeleton placeholders matching the component cards.

Changes

• Add responsive skeleton cards with reduced-motion support.
• Share styling through PcPartCardLayout.
• Show up to two skeleton rows on desktop.
• Preserve existing cards while loading more results.

Related issues

None.

Verification

• Build and lint passed.
• All 25 tests passed, including loading and pagination checks.
• Manual visual verification pending.

Screenshots

Checklist

• I have reviewed my own changes.
• The changes meet the linked issue's acceptance criteria, where applicable.
• I have verified the changes and recorded the results above.
• I have added or updated tests where needed.
• I have updated documentation where needed.
• I have included screenshots for visual changes, or marked them not applicable.

[wilhelsr]

LGTM, the skeleton loading improvements look good and all checks pass—approved!