Reorganize source structure for better maintainability

- Move from 3-file structure to organized 5-file structure
- Create css/ and js/ subdirectories for better organization
- Split styles into main.css and themes.css for clarity
- Extract configuration constants to separate config.js file
- Rename template.html to index.html for clarity
- Update build.js to handle new organized structure
- Replace magic numbers with CALCULATOR_CONFIG constants

New structure:
  src/
    ├── index.html       (HTML template)
    ├── css/
    │   ├── main.css     (Core styles)
    │   └── themes.css   (Theme variations)
    └── js/
        ├── config.js    (Configuration constants)
        └── calculator.js (Main logic)

This provides a good balance between organization and simplicity,
making the codebase easier to maintain without over-modularization.

iframe.html

@@ -448,7 +448,8 @@
         }
 
         /* Dark theme - manual override */
-        .dog-calculator-container.theme-dark {
+
+.dog-calculator-container.theme-dark {
             --bg-primary: #24202d;
             --bg-secondary: #312b3b;
             --border-color: #433c4f;
@@ -2024,6 +2025,18 @@
     </div>
     <script>
         /**
+ * Configuration constants for Dog Calorie Calculator
+ */
+
+const CALCULATOR_CONFIG = {
+    defaultTheme: 'system',
+    defaultScale: 1.0,
+    maxFoodSources: 5,
+    minScale: 0.5,
+    maxScale: 2.0
+};
+
+/**
          * Dog Calorie Calculator - iframe version
          * by Canine Nutrition and Wellness
          */
@@ -2032,10 +2045,10 @@
             constructor() {
                 this.currentMER = 0;
                 this.isImperial = false;
-                this.theme = this.getThemeFromURL() || 'system';
-                this.scale = this.getScaleFromURL() || 1.0;
+                this.theme = this.getThemeFromURL() || CALCULATOR_CONFIG.defaultTheme;
+                this.scale = this.getScaleFromURL() || CALCULATOR_CONFIG.defaultScale;
                 this.foodSources = [];
-                this.maxFoodSources = 5;
+                this.maxFoodSources = CALCULATOR_CONFIG.maxFoodSources;
                 this.init();
             }
 
@@ -2061,7 +2074,7 @@
             getScaleFromURL() {
                 const urlParams = new URLSearchParams(window.location.search);
                 const scale = parseFloat(urlParams.get('scale'));
-                return (!isNaN(scale) && scale >= 0.5 && scale <= 2.0) ? scale : null;
+                return (!isNaN(scale) && scale >= CALCULATOR_CONFIG.minScale && scale <= CALCULATOR_CONFIG.maxScale) ? scale : null;
             }
 
             applyTheme() {
@@ -2074,8 +2087,8 @@
                 const container = document.getElementById('dogCalculator');
                 if (!container) return;
 
-                // Clamp scale between 0.5 and 2.0 for usability
-                const clampedScale = Math.max(0.5, Math.min(2.0, this.scale));
+                // Clamp scale between min and max for usability
+                const clampedScale = Math.max(CALCULATOR_CONFIG.minScale, Math.min(CALCULATOR_CONFIG.maxScale, this.scale));
                 
                 if (clampedScale !== 1.0) {
                     container.style.transform = `scale(${clampedScale})`;