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>

.env.example

@@ -1,20 +1,25 @@
 # Flask Configuration
-FLASK_SECRET_KEY=your_secret_key_here
+FLASK_SECRET_KEY=calendar-display-secret-key-change-this
 
 # OpenWeatherMap API
-OPENWEATHER_API_KEY=your_api_key_here
+# Get your free API key from: https://openweathermap.org/api
+OPENWEATHER_API_KEY=your_openweather_api_key_here
 
-# Weather Location (Hamilton, New Zealand)
-WEATHER_LOCATION=Hamilton,NZ
-WEATHER_LAT=-37.7870
-WEATHER_LON=175.2793
+# Weather Location
+# Find coordinates at: https://www.latlong.net/
+WEATHER_LOCATION=YourCity,CountryCode
+WEATHER_LAT=0.0000
+WEATHER_LON=0.0000
 
 # Google Calendar
+# Option 1: Use public iCal URL (easiest - no authentication needed)
+# Get from: Google Calendar Settings > Calendar > Integrate Calendar > Public URL to this calendar (iCal format)
 GOOGLE_CALENDAR_ID=your_calendar_id@group.calendar.google.com
+GOOGLE_CALENDAR_ICAL_URL=https://calendar.google.com/calendar/ical/your_calendar_id%40group.calendar.google.com/public/basic.ics
 
 # Update Intervals (seconds)
-IMAGE_ROTATION_INTERVAL=300
+IMAGE_ROTATION_INTERVAL=60
 WEATHER_UPDATE_INTERVAL=900
 CALENDAR_UPDATE_INTERVAL=300
 JOKE_UPDATE_INTERVAL=3600
-CALENDAR_DAYS_AHEAD=7
+CALENDAR_DAYS_AHEAD=5