Rename everything to Stemify, and add a README

- Python package stemroller/ -> stemify/, Android package com.stemroller.mobile ->
  com.stemify.mobile, app names, QML module, icons, desktop entries, MPRIS and Plex
  product names
- Done-markers are now .stemify.json; stemd renames the old ones once at startup so
  songs separated before the rename aren't separated again
- A top-level README covering the server, the Android and Android Auto app, the EQ,
  Stem Now and the desktop apps

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

android/settings.gradle.kts

@@ -19,5 +19,5 @@ dependencyResolutionManagement {
     }
 }
 
-rootProject.name = "StemRoller"
+rootProject.name = "Stemify"
 include(":app")