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/app/build.gradle.kts

@@ -8,11 +8,11 @@ plugins {
 }
 
 android {
-    namespace = "com.stemroller.mobile"
+    namespace = "com.stemify.mobile"
     compileSdk = 36
 
     defaultConfig {
-        applicationId = "com.stemroller.mobile"
+        applicationId = "com.stemify.mobile"
         minSdk = 33
         targetSdk = 36
         versionCode = 1