fix(app): the subtitle preview is drawn in the chosen font

The preview drew every font in the app's own Inter, so picking one changed
nothing on screen. On Linux it now asks fontconfig for the face libass would
use — the family with the weight and slant wanted, or fontconfig's fallback
when it is not installed — loads it on a thread of its own and draws the
sample in it. A bold or italic the face lacks is made up, as libass does.
Windows and macOS have no fc-match and keep the app's font, as the hint there
still says.

The CJK fallback and the subtitle face are now both kept, so installing one
no longer drops the other.

docs/USER_GUIDE.md

@@ -196,8 +196,9 @@ How subtitles look. A sample line at the top shows the style on a bright and a d
 a video that is playing changes at once.
 
 - **Size** and **Position**: how big the line is and how far up the picture it sits.
-- **Font**, **Bold** and **Italic**: any font installed on this computer. The sample line uses the
-  app's own font; the video uses the one you chose.
+- **Font**, **Bold** and **Italic**: any font installed on this computer. On Linux the sample
+  line shows the font too; on Windows and macOS it is drawn in the app's own font, and only the
+  video uses the one you chose.
 - **Text colour**, and a **Background** of **Outline** or **Box**, each with its own colour. A
   colour's opacity sets how much of the picture shows through. Choose **Box** if text is hard to
   read on bright scenes.