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>

TROUBLESHOOTING.md

@@ -243,6 +243,33 @@ See `TEMPLATE_V2_GUIDE.md` for comprehensive template documentation.
 
 ---
 
+## Project Files Not Auto-Loading
+
+### Problem Description
+
+**Symptoms:**
+- Uploaded project.json but CSV/SVG/CSS files don't load
+- Have to manually select each file individually
+- Timeline doesn't render after selecting project.json
+
+### Solution
+
+This is expected behavior due to browser security restrictions. When you upload a single project.json file, the browser doesn't allow automatic access to other files in the same directory.
+
+**Use the Folder Picker (Recommended):**
+
+1. Click **"📂 Load Project Folder"** (blue button at top of file manager)
+2. Select the entire project folder
+3. All files will be loaded automatically
+
+This works because you're explicitly granting permission to access all files in the folder.
+
+**Alternative:** Load files individually using the separate upload buttons for CSV, SVG, and CSS.
+
+**For developers running from local server:** Files auto-load when served via HTTP (no manual upload needed).
+
+---
+
 ## CSV Data Not Loading
 
 ### Problem Description