feat(desktop): version source-of-truth + RPM + AppImage packaging

Phase 1+2 of multi-platform distribution plan:
- gradle/libs.versions.toml: add `app = "1.08.0"` as single source of truth
- Root build.gradle: allprojects { version = libs.versions.app.get() }
- amethyst/build.gradle: versionName from catalog (versionCode stays local)
- desktopApp/build.gradle.kts: drop hardcoded "1.0.0"; inherit project.version;
  add TargetFormat.Rpm; add linux DSL (menuGroup, appCategory, debMaintainer,
  rpmLicenseType, rpmPackageVersion with dashes stripped)
- desktopApp/build.gradle.kts: new createReleaseAppImage task wrapping
  createReleaseDistributable with linuxdeploy (TargetFormat.AppImage is
  broken in Compose 1.10.x — CMP-7101)
- packaging/appimage/: AppRun launcher (sets LD_LIBRARY_PATH for bundled VLC),
  amethyst.desktop XDG entry, 512x512 icon extracted from icon.icns
- scripts/asset-name.sh: single source for release asset naming contract

gradle/libs.versions.toml

@@ -1,4 +1,8 @@
 [versions]
+# Amethyst app version — single source of truth consumed by both Android (amethyst/)
+# and Desktop (desktopApp/). Android versionCode is bumped independently in
+# amethyst/build.gradle because it must be a monotonic integer.
+app = "1.08.0"
 accompanistAdaptive = "0.37.3"
 cachemapVersion = "0.2.4"
 composeMultiplatform = "1.10.3"