feat: adopt existing Codex installs

[Wangnov]

Summary

• add a quiet manual picker for adopting an existing Codex install when Home cannot detect Codex
• validate and record user-selected macOS/Windows installs, with Windows provenance preferring the latest adopted record
• normalize install/action error copy so raw engine install errors do not leak into the main banner
• make Windows portable updates force-close only target-root Codex processes after graceful close fails
• bump Manager to v0.2.4 and add release notes

Validation

• npm run check
• npm run test
• npm run build
• cargo test --manifest-path src-tauri\Cargo.toml --workspace
• cargo clippy --manifest-path src-tauri\Cargo.toml --workspace --all-targets -- -D warnings
• git diff --check
• npm run tauri:build