PM-6311: Remove My Submissions desktop scrollbar

[jmgasper]

Related JIRA Ticket:

https://topcoder.atlassian.net/browse/PM-6311

What's in this PR?

• compacts the Marathon Match My Submissions columns and cell spacing
• removes the unnecessary horizontal scrollbar at the desktop design width
• preserves contained overflow on narrower table layouts and the mobile record-card layout
• adds a regression assertion and updates the Opportunities documentation

Validation

• focused Opportunities flow tests: 61/61 passed
• full lint passed
• TypeScript no-emit check passed
• production build passed with an 8 GB heap (existing warnings only)
• Chromium layout checks confirmed equal client and scroll widths at the reported desktop size and on mobile
• git diff --check passed