Fresh installs and new profiles now default to the Fusion (dark) theme
instead of the platform-native theme. The old "Default" theme is
renamed "System" to better describe its purpose — using the OS-native
Qt style (windowsvista, macOS, etc.).
Existing users who had "Default" selected are automatically migrated
to "System" so they keep their platform-native styling. Users with
an empty or invalid theme name now fall back to Fusion.
- PaletteEditorDialog::onSave(): compare whole PaletteConfig (colors and
appColors) so a change to only AccentStrong/AccentSoft writes the file;
add PaletteConfig::operator==.
- appColor(): derive both roles from QPalette::Highlight unconditionally.
The Fusion palettes pin Accent to near-Window values, and QPalette::Accent
only exists on Qt 6.6+, so keying on it made identical themes render very
differently across Qt versions.
- themeChangedSlot(): merge the theme default's [AppColors] into a custom
palette that predates the section instead of all-or-nothing per file;
hasPalette() now counts an appColors-only file as a palette.
- Add Default/palette-default-light.toml so the Default theme's Light scheme
keeps the classic greens instead of falling back to the OS accent.
- home_widget: restore the isBuiltInTheme() half of the Automatic condition;
non-built-in themes extract button colors from their own background art.
- palette_grid_widget: use appEnum.value(i) for the role cast (3 sites),
append appHeader to headerLabels, fix the 'Lighted' typo.
