fix: improve mobile purchase actions

[apesch85]

What does this PR do?

• Keeps the inventory row action menu inside the mobile viewport and expands the three-dot touch target.
• Makes copy purchase-price editing full-width on mobile with a sticky save footer and explicit save-error handling.
• Changes Field Mode Bought It into an explicit purchase confirmation that asks for price paid, quantity, and condition.
• Clarifies the difference between an existing catalog/price record and an owned copy.

Testing

• npm run lint
• npx tsc --noEmit
• npm test -- --run — 445/445 passed
• npm run build
• node scripts/release-smoke.mjs
• Puppeteer mobile viewport: menu fully visible/fixed, edited price persisted in PUT payload, Field purchase price persisted in purchase payload

Type of Change

• Bug fix

Reported by Ded.Gamer in Discord.