fix(installer): write log file as UTF-8 so non-ASCII chars don't error

On Windows, `logging.FileHandler` opens the log file with
`locale.getpreferredencoding()` when no encoding is passed — cp1252 on
a US-English install. Lines containing non-ASCII glyphs like ✓ (used in
prereq-status output) hit UnicodeEncodeError on emit; the logging
module catches it and prints `--- Logging error ---` plus a traceback
to stderr. The install still completes, but the noise is alarming.

Add `"encoding": "utf-8"` to the file-handler dictConfig so the file is
opened in UTF-8 regardless of locale.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: aarthy-dk

dk-installer.py

@@ -684,6 +684,10 @@ def configure_logging(self, debug=False):
                         "class": "logging.FileHandler",
                         "filename": str(file_path),
                         "formatter": "file",
+                        # Default is locale.getpreferredencoding(), which is
+                        # cp1252 on US Windows — chokes on non-ASCII chars like
+                        # ✓ that the installer prints in prereq status lines.
+                        "encoding": "utf-8",
                     },
                     "console": {
                         "level": "DEBUG",