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/proguard-rules.pro

@@ -1,10 +1,10 @@
 # kotlinx.serialization keeps its generated serializers via its own consumer rules.
 -keepattributes *Annotation*, InnerClasses
 -dontnote kotlinx.serialization.**
--keep,includedescriptorclasses class com.stemroller.mobile.**$$serializer { *; }
--keepclassmembers class com.stemroller.mobile.** {
+-keep,includedescriptorclasses class com.stemify.mobile.**$$serializer { *; }
+-keepclassmembers class com.stemify.mobile.** {
     *** Companion;
 }
--keepclasseswithmembers class com.stemroller.mobile.** {
+-keepclasseswithmembers class com.stemify.mobile.** {
     kotlinx.serialization.KSerializer serializer(...);
 }