Reduce Decodex App refresh chatter

[yvette-carlisle]

Summary

• cache Decodex App server liveness briefly and retry once after transport failures instead of probing /livez before every account request
• start bundled App servers as decodex serve --api-only --listen-address ... without --interval, and reject serve --api-only --interval in the CLI/runtime
• keep API-only serve as HTTP/API-only with one initial dashboard snapshot and no project polling loop
• align docs with the local account-control API on the Decodex server surface

Verification

• swift build --package-path apps/decodex-app
• cargo make fmt-rust
• cargo make check-rust
• cargo make check-vstyle-rust
• cargo test -p decodex api_only -- --nocapture
• cargo test -p decodex serve_ -- --nocapture
• cargo test -p decodex operator_state_endpoint_serves_account_api_snapshot -- --nocapture
• cargo make test-decodex-app-stage
• /Users/x/.codex/plugins/cache/hack-ink/playbook/0.3.0/scripts/semantic_drift_audit.py with agent verdict: pass