Add `get_fonts()` API and fix `.px_size` for non-Qt ctxs

Add a public `.ui._style.get_fonts()` helper to retrieve the
`_font[_small]: DpiAwareFont` singleton pair. Adjust
`DpiAwareFont.px_size` to return `conf.toml` value when Qt returns `-1`
(no active Qt app).

Also,
- raise `ValueError` with detailed msg if both Qt and a conf-lookup fail
- add some more type union whitespace cleanups: `int | None` -> `int|None`

(this commit-msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/ui/_style.py

@@ -107,7 +107,22 @@ class DpiAwareFont:
 
     @property
     def px_size(self) -> int:
-        return self._qfont.pixelSize()
+        size: int = self._qfont.pixelSize()
+
+        # XXX, when no Qt app has been spawned this will always be
+        # invalid..
+        # SO, just return any conf.toml value.
+        if size == -1:
+            if (conf_size := self._font_size) is None:
+                raise ValueError(
+                    f'No valid `{type(_font).__name__}.px_size` set?\n'
+                    f'\n'
+                    f'-> `ui.font_size` is NOT set in `conf.toml`\n'
+                    f'-> no Qt app is active ??\n'
+                )
+            return conf_size
+
+        return size
 
     def configure_to_dpi(self, screen: QtGui.QScreen | None = None):
         '''
@@ -221,6 +236,20 @@ def _config_fonts_to_screen() -> None:
     _font_small.configure_to_dpi()
 
 
+def get_fonts() -> tuple[
+    DpiAwareFont,
+    DpiAwareFont,
+]:
+    '''
+    Get the singleton font pair (of instances) from which all other
+    UI/UX should be "scaled around".
+
+    See `DpiAwareFont` for (internal) deats.
+
+    '''
+    return _font, _font_small
+
+
 # TODO: re-compute font size when main widget switches screens?
 # https://forum.qt.io/topic/54136/how-do-i-get-the-qscreen-my-widget-is-on-qapplication-desktop-screen-returns-a-qwidget-and-qobject_cast-qscreen-returns-null/3