Generalize code and improve documentation

Remove personal information and improve setup documentation:

**Code Generalization:**
- Updated .env.example with placeholder values
- Removed personal API keys and calendar IDs
- Removed all personal background images (15 images deleted)
- Added backgrounds directory README with usage instructions
- Updated .gitignore to exclude background images

**Documentation Improvements:**
- Added comprehensive Weather API setup instructions
  - How to get OpenWeatherMap API key
  - How to find location coordinates using LatLong.net
- Added detailed Google Calendar setup instructions
  - Public iCal feed method (easiest, no auth required)
  - Alternative Google Calendar API method
- Added background image specifications
  - Supported formats, recommended resolution
  - Where to add images and how to change rotation interval
- Added configuration section explaining all settings
  - Environment variables documentation
  - JavaScript intervals configuration
  - How to change image rotation time in both .env and app.js
- Added dynamic text color feature to feature list

**Setup Instructions Now Include:**
- Step-by-step Weather API key acquisition
- Google Calendar public iCal URL setup
- Background image guidelines and rotation configuration
- All configuration options clearly documented

Users can now easily set up their own calendar display without any personal information in the codebase.

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

static/backgrounds/README.md

@@ -0,0 +1,48 @@
+# Background Images
+
+This directory is where you place your background images for the calendar display.
+
+## Adding Images
+
+1. Copy your image files to this directory:
+   ```bash
+   cp /path/to/your/images/* static/backgrounds/
+   ```
+
+2. Supported formats:
+   - JPEG/JPG
+   - PNG
+   - GIF
+   - WebP
+
+3. Recommended image specifications:
+   - Resolution: 1920x1080 or higher
+   - Aspect ratio: 16:9 (for full-screen displays)
+   - File size: Keep under 5MB for faster loading
+
+## Image Rotation
+
+Images automatically rotate at the interval specified in your `.env` file.
+
+To change the rotation interval, edit the `.env` file:
+
+```bash
+# Time in seconds (default: 60 seconds = 1 minute)
+IMAGE_ROTATION_INTERVAL=60
+```
+
+Or directly in `static/js/app.js`:
+
+```javascript
+const INTERVALS = {
+    BACKGROUND: 60000,  // Milliseconds (60000 = 1 minute)
+    ...
+};
+```
+
+## Tips
+
+- Use high-quality images for best display
+- The app will randomly select images from this directory
+- Delete or move images you no longer want displayed
+- Images are cached by the browser, so you may need to hard-refresh (Ctrl+F5) to see new images immediately