Polish Decodex app login and fast mode controls

[yvette-carlisle]

Summary

• add a Decodex App fast mode toggle backed by ~/.codex/config.toml
• polish login into a popup panel with clearer device-code feedback
• refine app controls and server attach/start behavior

Validation

• cargo +nightly fmt --all -- --check
• swift build --package-path apps/decodex-app
• cargo test -p decodex fast_mode
• cargo test -p decodex parses_fast_mode_set_bridge_request
• cargo check -p decodex
• cargo make test-decodex-app-stage
• git diff --check