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>

.gitignore

@@ -43,3 +43,8 @@ Thumbs.db
 
 # Logs
 *.log
+
+# Background Images (users should add their own)
+static/backgrounds/*
+!static/backgrounds/README.md
+!static/backgrounds/.gitkeep