feat: improve mobile touch controls and add local file drop support

fix: missing useThree and THREE imports for CameraLogger

fix: syntax error in WorldViewer.tsx

feat: add developer P key logger and remove PlacementEditor

style: hide background canvas during upload and use solid black overlay

feat: implement concurrent chunked upload and JYSM prefix URL routing

refactor: extract UploadOverlay into separate component

feat: enable mobile touch joystick in fly mode

fix: update github link and remove obsolete terminal button

fix: update operation hints to accurately reflect controls

fix: restore accidentally deleted icon in BottomLeftControls

style: remove emoji from operation hint

feat: move operation hints to persistent bottom left controls

feat: fix white flash on load and add operation hints to loading screen

Initial commit: SparkSplat with proper UTF-8 LFS