Improve build system and clean up project structure

- Configure build script to create backups in backup/ directory
- Add backup/ directory to .gitignore to keep repo clean
- Add .gitignore file with common exclusions
- Remove obsolete dog-food-calculator-widget.js file
- Move all temporary and reference files to sundog-calculator-meta/

This keeps the project root clean and organized while maintaining
automatic backups during builds.

.gitignore

@@ -0,0 +1,27 @@
+# Dependencies
+node_modules/
+
+# IDE
+.vscode/
+.idea/
+
+# OS
+.DS_Store
+Thumbs.db
+
+# Temporary files
+*.tmp
+*.temp
+
+# Personal files
+theme.scss
+_variables.scss
+reference.png
+better.png
+START_PROMPT.md
+CLAUDE.md
+CLAUDE_V2.md
+math.md
+vetcalculators/
+*.js.backup*
+backup/