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>

README.md

@@ -102,8 +102,9 @@ Just open `index.html` in any modern browser (Chrome, Firefox, Safari, Edge).
 
 You can choose between:
 
-* automatic loading from `project.json` in the project folder
-* manual upload of a `project.json` file via the UI
+* **Automatic loading** - When served via HTTP, the app auto-loads from `project.json` in project folders (binect/, my-project/, or example/)
+* **Load Project Folder** - Click "📂 Load Project Folder" and select an entire project directory. All files (project.json, CSV, CSS, SVG) will be loaded automatically
+* **Load files individually** - Upload project.json first, then manually upload CSV, CSS, and SVG files using the individual file buttons
 
 ### **3. Preview the timeline**