Rewrite CEF browser as child of Kivy window (v2)

v1 created a separate Win32 window — same problem as external Chrome.
v2 creates CEF as a CHILD WINDOW of Kivy's SDL_app window:
  - No separate taskbar entry
  - No z-order fighting (CEF is INSIDE Kivy)
  - No desktop flash
  - CEF message loop pumped via Kivy Clock (main thread)
  - Resize handler attached so CEF follows Kivy window changes
  - build.spec includes cef_browser in hidden imports

windows/build.spec

@@ -102,6 +102,7 @@ hidden_imports = [
     'selectors',
     'tempfile',
     # Windows-specific
+    'cef_browser',
     'win32gui',
     'win32con',
 ]
@@ -134,9 +135,7 @@ if kv_file.exists():
     kv_data.append((str(kv_file), '.'))
 
 # Bundle the entire src directory as a tree
-source_tree = Tree(str(SRC_DIR), prefix='')
-# Filter out pyc files, pycache dirs, and ini files
-source_tree.excludes = ['*.pyc', '__pycache__', '*.ini']
+source_tree = Tree(str(SRC_DIR), prefix='', excludes=['*.pyc', '__pycache__', '*.ini'])
 
 # --- Collect binary DLLs from kivy_deps and ffpyplayer ----------------
 import importlib.util