[Themes] Encode home backgrounds as JPEG to slim the repo (#7346)

The home background PNGs shipped since #7280 total ~67 MB and bloat
every clone and pull for other devs. Encode them as JPEG at quality 95
(matching resolution, visually lossless at 40-48 dB PSNR since the art
is noise-heavy renders PNG cannot compress) for ~79% size reduction.

The scheme-variant and theme asset resolution already probe .jpg, so
only the qrc entries needed updating.

Refs #7327

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

cockatrice/cockatrice.qrc

@@ -62,9 +62,9 @@
         <file>resources/icons/mana/U.svg</file>
         <file>resources/icons/mana/W.svg</file>
 
-        <file>resources/backgrounds/home.png</file>
+        <file>resources/backgrounds/home.jpg</file>
-        <file>resources/backgrounds/home-dark.png</file>
+        <file>resources/backgrounds/home-dark.jpg</file>
-        <file>resources/backgrounds/home-light.png</file>
+        <file>resources/backgrounds/home-light.jpg</file>
         <file>resources/backgrounds/card_triplet.svg</file>
         <file>resources/backgrounds/placeholder_printing_selector.svg</file>