raise webxdc sending limit to 640 kb

as discussed in dev chat,
100 kb is too low if one wants to use any game framework.
also, many game that doesn't have poor graphics
or uses magic CSS-tricks to do fancy draws,
will have more than 100 kb probably just in tiles and sprites.

even for regions with low resources,
100 kb is low for a game -
esp. if we compare that with stickers that are send around as well
are not deduplicated and also have several 100 kb in size -
for much less effect.

we should still encourage ppl to create tiny apps,
however, a too low limit also restricts possibilities wrt adaption.

src/webxdc.rs

@@ -32,12 +32,12 @@ const WEBXDC_DEFAULT_ICON: &str = "__webxdc__/default-icon.png";
 ///
 /// The limit is also an experiment to see how small we can go;
 /// it is planned to raise that limit as needed in subsequent versions.
-const WEBXDC_SENDING_LIMIT: usize = 102400;
+const WEBXDC_SENDING_LIMIT: usize = 655360;
 
 /// Be more tolerant for .xdc sizes on receiving -
 /// might be, the senders version uses already a larger limit
 /// and not showing the .xdc on some devices would be even worse ux.
-const WEBXDC_RECEIVING_LIMIT: usize = 1048576;
+const WEBXDC_RECEIVING_LIMIT: usize = 4194304;
 
 /// Raw information read from manifest.toml
 #[derive(Debug, Deserialize)]