Update AppStream metadata for Linux

Flathub seems to be more strict about recent formats

dist/linux/app.lith.Lith.appdata.xml.in

@@ -1,6 +1,8 @@
-<?xml version="1.0" encoding="utf-8"?>
-<component>
-    <id type="desktop">app.lith.Lith.desktop</id>
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Copyright 2024 Martin Bříza -->
+<component type="desktop-application">
+    <id>app.lith.Lith.desktop</id>
+    <launchable type="desktop-id">app.lith.Lith.desktop</launchable>
     <project_license>GPL-2.0+</project_license>
     <metadata_license>CC0-1.0</metadata_license>
     <name>Lith</name>
@@ -15,8 +17,14 @@
     </keywords>
     <url type="homepage">https://lith.app</url>
     <screenshots>
-        <screenshot type="default">https://lith.app/assets/main.png</screenshot>
+        <screenshot type="default">
+            <image>https://lith.app/assets/main.png</image>
+            <caption>Default chat window</caption>
+        </screenshot>
     </screenshots>
+    <developer id="cz.rtinbriza.ma">
+        <name>Martin Bříza</name>
+    </developer>
     <update_contact>m@rtinbriza.cz</update_contact>
     <content_rating type="oars-1.1"/>
     <releases>