fix: rename main_md.py → main.py so buildozer uses correct entry point

buildozer ignores source.main setting and always uses main.py as entry point.
Renamed main_md.py (KivyMD UI) to main.py and old main.py to main_classic.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: samvelkoch

android_app/buildozer.spec

@@ -4,7 +4,6 @@ package.name    = dungeonblackcastlemd
 package.domain  = org.braslavsky
 
 source.dir      = .
-source.main     = main_md.py
 source.include_exts = py,json,ttf,png
 source.include_patterns = assets/*,icons/*,sections.json,TwemojiMozilla.ttf,ArialUnicode.ttf