Refactor Android wallet and model organization

[callebtc]

Summary

• Split Android's monolithic Models.kt into focused Payments, Mints, Quotes, Requests, Settings, Tokens, Transactions, and WalletSupport model files.
• Move WalletManager into Core/Wallet and extract wallet state, mint metadata fetching, mint quote sync, transaction loading, and database recovery helpers.
• Keep Kotlin package names stable so existing imports and serialization references continue to resolve.

Verification

• git diff --cached --check before commit
• static source checks for conflict markers, duplicate declarations, and stale Models.kt/Core/WalletManager.kt references
• Android Gradle compile not run: this environment has no Java runtime installed or selected.

Note

This branch was intentionally created from PR #67's resolved branch. Until #67 merges, this PR's full diff against main also includes the Swift organization refactor from that branch.