feat: add cross-platform background run modes

[bradtraversy]

Summary

• add a terminal-only foreground or background launch choice for plain portdoc
• add explicit background start, stop, status, and disable commands
• manage one stable per-user entry through systemd on Linux, launchd on macOS, and Task Scheduler on Windows
• add bounded manager commands, health verification, conflict handling, atomic definition and config writes, and persisted background ports
• document the new commands, native managers, and current upgrade limitation

Safety

PortDoc manages only its stable native entry: portdoc.service, com.traversymedia.portdoc, or PortDoc. Lifecycle commands never stop a process selected from a listener PID. Background start refuses a foreground PortDoc or unrelated listener on the target port.

Validation

• cargo fmt --check
• cargo test (146 passed)
• cargo build
• cargo clippy -- -D warnings
• cargo check --tests --target x86_64-pc-windows-msvc
• npm run lint
• npm run build
• git diff --check
• GitHub Actions CI passed on Ubuntu, macOS, and Windows for both push and pull-request runs on the implementation commit

Live evidence and accepted gaps

• Linux foreground, background start, status, terminal detachment, stop, and disable passed live.
• Brad reported the macOS foreground and background lifecycle working correctly.
• The Windows source build stopped before compiling PortDoc because the machine's active GNU Rust toolchain lacked dlltool. Native Windows CI passed the feature tests and strict Clippy.
• Sign-in startup was not manually tested on any platform.

Brad accepted the Windows live-test and sign-in-test gaps on July 27, 2026, and approved proceeding on the automated and available live evidence. These are recorded verification gaps, not passing manual evidence.