build: implement Flatpak packaging and update repository references

- Add `flatpak-sources.json` containing a comprehensive list of offline Maven/Gradle dependencies for Flatpak builds.
- Configure `flatpak-gradle-generator` plugin in `build.gradle.kts` to automate source manifest generation.
- Update Flatpak manifest (`zed.rainxch.githubstore.yml`) and metainfo with correct SHA-256 hashes for JBR and Gradle distributions.
- Update project URLs and VCS references from `AshkanAhmady/GitHub-Store` to `OpenHub-Store/GitHub-Store`.

Author: rainxchzed

build.gradle.kts

@@ -1,4 +1,5 @@
 plugins {
+    id("io.github.jwharm.flatpak-gradle-generator") version "1.7.0"
     alias(libs.plugins.android.application) apply false
     alias(libs.plugins.android.library) apply false
     alias(libs.plugins.compose.hot.reload) apply false
@@ -11,6 +12,10 @@ plugins {
     alias(libs.plugins.room) apply false
 }
 
+tasks.named<io.github.jwharm.flatpakgradlegenerator.FlatpakGradleGeneratorTask>("flatpakGradleGenerator") {
+    outputFile.set(layout.buildDirectory.file("flatpak-sources.json"))
+}
+
 subprojects {
     afterEvaluate {
         tasks.configureEach {