search redesign: apply search results QA fixes

Created by: limitedmage

Fixes #21153 (closed)

• change 'repository name match' to 'repository match'
• align sidebar collapse chevron icon with margin
• add 0.25rem extra margin between search results
• add rounded corners to search progress indicator
• tighten spacing in sidebar responsive layouts

Notes:

• Icon size has been addressed by #21083
• Icon color has been addressed by #21180
• Tooltips have been addressed by #21362
• Diff/commit styling has been addressed by #21213