[TheDrOnos]

Fixes #31.

In an offline install, the interpreter step is uv python install <ver> with UV_PYTHON_INSTALL_MIRROR pointing at the wheelhouse's python/ folder. uv words that copy as a download, and runUv relayed its output unchanged. So the setup window showed "Downloading cpython-3.13.12-…" as its step, with the Download pill lit, although nothing is downloaded.

Change (js/main/pythonEnv.ts, the wheelhouse branch of setupEnv only):

• The step is announced as [env-setup] installing Python <ver> from the wheelhouse, which the setup window shows as "Installing Python 3.13.12 from the wheelhouse".
• uv's output for that step goes through the new withoutPythonDownloadLines. It drops uv's "Downloading cpython-…" and "Downloaded cpython-…" lines and passes the rest, so the log keeps "Installed Python …".
• The filter works on whole lines. A chunk's unfinished last line is held until its end arrives or the step ends, so a line split across chunks is still dropped whole.
• The online path is unchanged: there uv's wording is true.

Tests (pythonEnv.test.ts), on uv 0.13.4's real output for this step:

• the download lines are dropped and the rest kept;
• a download line split across chunks is dropped whole;
• an unfinished last line still reaches the stream at flush;
• through parseUvLine, the step reads "Installing Python 3.13.12 from the wheelhouse", and no event is in the downloading phase.

With the filter disabled, three of them fail: the first, second and fourth. All four failed before the function existed.

Checks:

• npm run typecheck: clean.
• npm run test:unit: 179 passed.

Not verified:

• A packaged offline install end to end.
• Other uv versions' wording for this step. The filter matches uv 0.13.4's "Downloading/Downloaded cpython-".
• The output of the later offline steps (venv, export, pip install), as in the issue.

The changelog fragment is upcoming_changes/32.bugfix.rst, added in a second commit once the PR had its number, as #20 did.