Phase 13: Squad mode — 4-player loadout builder

[elfensky]

Summary

Stretch goal. Extend the loadout builder to support 4-player squads.

Parent: #162

Design

• Hash format: [[player1...], [player2...], ...] detected by checking if arr[0] is an array
• Player tabs (Player 1-4) with editable names
• Solo/Squad toggle switch
• Compact summary of all 4 loadouts at bottom
• Shared squad URLs show vertical layout
• OG image renders 2x2 grid

Files

• CREATE src/features/loadout/SquadBuilder.jsx (~250-350 LOC)
• CREATE src/features/loadout/SquadSummary.jsx
• MODIFY src/shared/utils/loadoutHash.mjs — add squad encode/decode
• MODIFY src/__tests__/unit/shared/utils/loadoutHash.test.mjs
• MODIFY src/app/loadout/opengraph-image.jsx
• MODIFY src/app/loadout/page.jsx

Size: XL