feat: add folder picker for automatic project file loading

- Add "Load Project Folder" button with folder selection (webkitdirectory)
- Automatically load all project files (JSON, CSV, SVG, CSS) from selected folder
- Eliminate need to manually upload each file individually
- Show clear errors if referenced files are missing from folder
- Update all documentation to explain folder picker usage

This solves the browser security limitation where uploading a single
project.json doesn't allow automatic access to other files in the same
directory. Users can now select an entire project folder and all files
load automatically in one click.

Changes:
- index.html: Add folder input with webkitdirectory attribute and UI
- engine.js: Add folderInput event handler to process all files from folder
- README.md: Document folder picker as primary loading method
- WINDOWS_USAGE.md: Add folder picker as recommended Option 1
- TROUBLESHOOTING.md: Add section explaining project files not auto-loading
- CLAUDE.md: Document folder picker architecture for future instances
- Makefile: Update DIST_README.md template to mention folder picker

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -149,6 +149,13 @@ Use this to deploy the application to Windows or other environments. The distrib
 - Internal view: Shows item IDs (for development/review)
 - External view: Hides item IDs (for presentations/exports)
 
+**Project Loading Methods**:
+- Auto-load: When served via HTTP, attempts to load from binect/, my-project/, or example/ folders
+- Folder picker: User selects entire project folder - all files load automatically (uses webkitdirectory attribute)
+- Individual files: User manually uploads project.json, then CSV/SVG/CSS separately
+
+The folder picker was added to solve the browser security limitation where uploading a single project.json doesn't allow automatic access to other files in the same directory.
+
 ## Project Structure
 
 ```