fix(extensions): force UTF-8 stdio on Windows to stop UnicodeEncodeError crashes

[rekcilyssup]

Fixes#270 (UnicodeEncodeError during model auto-download) and the deadlock root cause described in #214.

Problem

On Windows, the embedded Python defaults stdout/stderr/stdin to the active console codepage (cp1252, cp932, ...). Two failure modes follow:

1. Crash (reported in Windows: UnicodeEncodeError during model auto-download #270):BaseGenerator._auto_download() prints → and …; on cp1252 stdout this raises UnicodeEncodeError: 'charmap' codec can't encode character '\u2192', aborting the model download for any extension using _auto_download() (e.g. triposplat).
2. Deadlock (reported in 3D generation hangs forever on Japanese Windows: extension worker deadlocks when stderr reader thread dies with UnicodeDecodeError (cp932) #214): the extension worker emits UTF-8 (and mixed-encoding tqdm output). The parent's Popen(text=True) pipe readers decode with the OS locale, so the stderr reader thread dies with UnicodeDecodeError on Japanese Windows; the stderr pipe fills and generation hangs forever at 0% CPU.

Changes

• api/services/generators/base.py — ASCII-ify the download progress print (-> / ...), the exact line from the Windows: UnicodeEncodeError during model auto-download #270 traceback.
• api/services/generator_registry.py — ASCII-ify the reload print (… -> ...), same crash class in the API process.
• api/services/stdio_utf8.py (new) — ensure_utf8_stdio() reconfigures stdin/stdout/stderr to UTF-8 with errors="replace", no-op when unsupported.
• api/main.py / api/runner.py — call ensure_utf8_stdio() at startup so both the FastAPI process and every extension worker emit UTF-8 regardless of locale.
• api/services/extension_process.py — spawn workers with PYTHONUTF8=1 and Popen(..., encoding="utf-8", errors="replace") so parent-side pipe readers never die on non-UTF-8 locales (the verified fix from 3D generation hangs forever on Japanese Windows: extension worker deadlocks when stderr reader thread dies with UnicodeDecodeError (cp932) #214).
• electron/main/process-runner.ts — set PYTHONUTF8=1 for process-extension spawns (same crash class).

Tests

• New api/tests/test_stdio_utf8.py (3 tests): helper reconfigures all three streams, skips streams without reconfigure, swallows errors.
• New api/tests/test_base_generator.py: regression test asserting the _auto_download print is ASCII-safe.
• api/tests/test_extension_process.py: asserts _build_env() sets PYTHONUTF8=1.

All 6 new tests pass. Node suite: 95/95 pass. Pre-existing failures on origin/dev (2 registry tests + fastapi import in system Python) are unchanged by this PR.