PR: Fix ExceptDialog hardcoded dependency and add cross-platform log file support - #48

Fixes two bugs and improves cross-platform compatibility across two files.

qtapputils/widgets/exceptions.py — ExceptDialog:

• Removes a hardcoded import of hydrogeolab.config.main.TEMP_DIR, replacing it with an optional temp_dir constructor parameter that validates the path and falls back to the system temp directory.
• Replaces os.startfile() (Windows-only) with QDesktopServices.openUrl() for cross-platform log file opening.
• Adds encoding='utf-8' when writing the log file.

qtapputils/managers/capture.py — StandardStreamEmitter:

• Adds a stream constructor parameter ('stdout' or 'stderr') so each emitter instance targets the correct underlying system stream.
• Updates write() to delegate to the configured stream instead of always writing to sys.__stdout__.
• Adds a flush() method that flushes the correct underlying stream, preventing AttributeError on sys.stdout.flush() or sys.stderr.flush() while capturing.