`_render_distribution_plan` reads the target profile's `.env` to decide
whether a required env var is already set (so it doesn't nag the user),
using `Path.read_text()` with no encoding. Two bugs:
1. `Path.read_text()` defaults to the system locale (cp1251/GBK on Windows),
which raises `UnicodeDecodeError` on any non-ASCII byte. The surrounding
`except OSError` does NOT catch that — `UnicodeDecodeError` is a
`ValueError` — so a mis-encoded `.env` aborts the entire install preview.
2. Even on a UTF-8 locale, a Notepad-added BOM prefixes the first key
(`KEY`), so the very first required env var is mis-reported as
"needs setting" when it is actually present.
`.env` is written as UTF-8 everywhere in the codebase. Read it as
`utf-8-sig` (tolerates the BOM) and also catch `UnicodeDecodeError` so a
genuinely un-decodable file skips the pre-check instead of crashing.
Regression tests: a BOM-prefixed `.env` whose first key must still read as
"set", and an invalid-UTF-8 `.env` that must not abort the preview.
