feat: add followed users and activity

[jo32]

Summary

• add follow and unfollow actions to user profiles
• add paginated followed-user and followed-user activity views
• add Rust services and protobuf models for NGA's follow_v2 APIs
• add Simplified Chinese localization and a parser regression test

Why

NGA supports following users and browsing their topic/reply activity, but MNGA did not expose those workflows. This adds the missing functionality using the existing Rust/Swift protobuf bridge and paging infrastructure.

The followed-user response wraps its records in data[0]; the parser handles that shape explicitly so valid follows are not filtered out.

Validation

• cargo test -p service --lib (10 passed, 46 network/manual tests ignored)
• cargo clippy -p service --all-targets
• make logic-sim
• iOS Simulator Debug build with code signing disabled

make swiftformat was also invoked, but the tool is not installed in the local environment, so the Makefile skipped it.

[jo32]