Add dynamic text color adjustment based on background brightness

Automatically adjusts text color for optimal readability:
- Bright backgrounds: Text switches to dark color (#1a1a1a)
- Dark backgrounds: Text remains white (#ffffff)
- Uses luminance formula (0.299×R + 0.587×G + 0.114×B) for accurate brightness detection
- Analyzes images using canvas to sample 100x100 pixels
- Smooth color transitions (0.5s) between background changes
- Adjusts text shadows for better contrast in each mode
- Event time color adapts (#0066cc for light, #4a9eff for dark)

Technical implementation:
- Added calculateImageBrightness() function in app.js
- Modified updateBackground() to analyze and apply appropriate CSS class
- Added .light-bg and .dark-bg CSS classes for text color themes
- Brightness threshold set at 140 (out of 255)

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

static/js/app.js

@@ -212,6 +212,56 @@ async function updateCalendar() {
     }
 }
 
+// Calculate brightness of an image
+function calculateImageBrightness(imageSrc, callback) {
+    const img = new Image();
+    img.crossOrigin = 'Anonymous';
+
+    img.onload = () => {
+        // Create a canvas to analyze the image
+        const canvas = document.createElement('canvas');
+        const ctx = canvas.getContext('2d');
+
+        // Use a smaller size for faster processing
+        canvas.width = 100;
+        canvas.height = 100;
+
+        // Draw the image scaled down
+        ctx.drawImage(img, 0, 0, 100, 100);
+
+        // Get image data
+        const imageData = ctx.getImageData(0, 0, 100, 100);
+        const data = imageData.data;
+
+        // Calculate average brightness
+        let totalBrightness = 0;
+        const pixelCount = data.length / 4;
+
+        for (let i = 0; i < data.length; i += 4) {
+            const r = data[i];
+            const g = data[i + 1];
+            const b = data[i + 2];
+
+            // Calculate perceived brightness (using luminance formula)
+            const brightness = (0.299 * r + 0.587 * g + 0.114 * b);
+            totalBrightness += brightness;
+        }
+
+        const avgBrightness = totalBrightness / pixelCount;
+
+        // Return brightness value (0-255)
+        callback(avgBrightness);
+    };
+
+    img.onerror = () => {
+        console.error('Error loading image for brightness analysis');
+        // Default to dark background if error
+        callback(100);
+    };
+
+    img.src = imageSrc;
+}
+
 // Update background image
 async function updateBackground() {
     try {
@@ -226,11 +276,28 @@ async function updateBackground() {
             const img = new Image();
             img.onload = () => {
                 backgroundElement.style.backgroundImage = `url('${data.image}')`;
+
+                // Analyze brightness and adjust text color
+                calculateImageBrightness(data.image, (brightness) => {
+                    // Threshold: 128 is middle brightness
+                    // If brightness > 140, use dark text (light background)
+                    // If brightness <= 140, use white text (dark background)
+                    if (brightness > 140) {
+                        document.body.classList.remove('dark-bg');
+                        document.body.classList.add('light-bg');
+                    } else {
+                        document.body.classList.remove('light-bg');
+                        document.body.classList.add('dark-bg');
+                    }
+                });
             };
             img.src = data.image;
         } else if (data.color) {
             backgroundElement.style.backgroundColor = data.color;
             backgroundElement.style.backgroundImage = 'none';
+            // Assume solid colors are dark
+            document.body.classList.remove('light-bg');
+            document.body.classList.add('dark-bg');
         }
 
     } catch (error) {