Optimize iPad layouts and fix Better Auth account schema

[Florian2807]

Summary

• Enable iPad support and adapt screen widths, floating controls, navigation titles, and the Record Payment sheet.
• Align split and payment controls, make allocation fields tappable across the existing gray field, and scroll a focused field only when the keyboard covers it.
• Fix Better Auth 1.7.5 account schema compatibility by making the legacy issuer column optional and restoring uniqueness on (provider_id, account_id). Existing issuer values are retained.

The archived Friends and Groups swipe work is in a separate local stash and is not included.

Verification

• pnpm publication:check
• pnpm audit --prod
• pnpm typecheck
• pnpm test - 177 mobile, 95 server, and 66 shared tests passed; the database package has no tests
• pnpm build - iOS, Android, and web exports passed
• git diff --check
• Physical-device checks completed when mobile layout or navigation changed

Physical-device checks remain for short and long lists, Profile and group settings, New Group and New Expense sheets, dragging populated and empty areas, light and dark mode, and the changed allocation controls. Android also needs touch, menu, sheet, Back, double-tap, and keyboard/scroll checks.

Security and privacy

• No credentials, production configuration, real user data, database dumps, private invitation links, or sensitive logs are included
• Authentication, authorization, retention, and privacy-policy implications were reviewed where applicable: the account migration preserves existing issuer data and restores the Better Auth account key
• No new dependencies or third-party assets