Reset Touch ID keychain storage

[0xfnzero]

Summary

• Reset desktop Touch ID wallet password storage to a clean v6 Keychain service.
• Remove legacy Touch ID service/account fallback and migration paths so old incompatible credentials no longer interfere.
• Keep Touch ID authentication explicit through macOS LocalAuthentication before reading the Keychain password.
• Tidy the desktop EVM custom chain editor behind a collapsible panel.

Testing

• cargo fmt --all -- --check
• cargo check -p fnzero-safe-desktop --all-features
• cargo test -p fnzero-safe-desktop --lib
• cargo clippy -p fnzero-safe-desktop --all-targets --all-features -- -D warnings
• npm run lint
• npm exec tsc -- --noEmit
• make package-macos

Notes

• Local stale Touch ID Keychain entries for old Fnzero/SafeKey services were removed on the development machine.
• Users must enable Touch ID once again after this change so the app writes the new v6 credential.