#1792 Suggest quality default fonts based on platform

[Hiroshi Miura]

Problem/Motivation
Currently, OmegaT sets the default font to the "Dialog" Java logical font. While this is suitable for menus and titles, it is not ideal for the text panes where translation work actually happens. "Dialog" often lacks the aesthetic quality and readability required for long-term text editing.

Modern operating systems come with high-quality fonts that are beautiful, readable, and offer wide support for various languages and locales. OmegaT should leverage these platform-specific standards to improve the user experience out of the box.

Proposed Solution
Introduce platform-specific logic to determine a sensible, high-quality default font family for the text editing areas, rather than relying on the generic "Dialog" font.

Implementation Idea
Improve the FontUtil class:

1. New Method: Added getPlatformDefaultFontFamily() to handle platform-specific font selection logic for Windows, macOS, and Linux/BSD/Unix-like environments.
2. Updated Methods: Modified getDefaultFont(), getConfiguredFontName(), and getFont() to use this new dynamic default instead of a hardcoded value.
3. Font Selection: Introduced a utility method pickFirstAvailable() to check a list of preferred candidate fonts against the GraphicsEnvironment and select the first one actually installed on the user's system.

Pull Request
PR #1764: https://github.com/omegat-org/omegat/pull/1764