[chloe41427]

Summary

Windows binaries must print paths with forward slashes, as on Unix. SDKMAN runs on Windows under a Unix-like shell, so backslashes have no place in its output. #461 started rendering paths with Path::display(), which uses \ on Windows. That broke the home output and the paths in error messages.

• Adds ui::slashed, which renders a path with / on every platform. It only replaces separators on Windows, because \ is a legal filename character on Unix.
• home prints its path through ui::slashed.
• ui::path, used for paths in messages, goes through ui::slashed.
• Tests expect / again: this reverts the native-separator assertion from a390df9 in tests/helpers.rs. tests/home.rs normalises the temp SDKMAN_DIR it compares against.
• Adds a unit test for ui::slashed.

Supersedes #464, which changed the tests to accept backslashes.

Test plan

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test (Linux arm64)
• Windows: pull_request.yml runs on pull_request_target and builds master, not this branch, so the push build on master after the merge is the Windows verification.