Fix cross-platform build: replace ${PROJECT_DIR} with relative paths

platformio.ini:
- Replace -I${PROJECT_DIR}/lib, -I${PROJECT_DIR}/deps/... with relative
  paths (-Ilib, -Ideps/...) in both tdeck-bluedroid and tdeck environments;
  ${PROJECT_DIR} is mangled on Windows inside build_flags, causing include
  paths to resolve inside the PlatformIO builder directory instead of the
  project root
- Remove hardcoded -I.pio/libdeps/tdeck/TinyGPSPlus/src and
  -I.pio/libdeps/tdeck/NimBLE-Arduino/src; these paths reference generated
  cache, break on fresh clones, and are redundant with lib_ldf_mode = deep+
- Fix OTA upload_command: replace python3 with $PYTHONEXE so it resolves
  to PlatformIO's bundled Python on Windows, macOS, and Linux

src/main.cpp, lib/tdeck_ui/UI/LXMF/UIManager.cpp:
- Change #include "tone/Tone.h" to #include "Tone.h"; PlatformIO
  automatically adds -Ilib/tone for local libraries, making the
  subdirectory prefix unnecessary and broken when -Ilib is not effective

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

src/main.cpp

@@ -58,7 +58,7 @@
 #include <UI/LXMF/SettingsScreen.h>
 
 // Audio notifications
-#include "tone/Tone.h"
+#include "Tone.h"
 
 // Logging
 #include <Log.h>