Add config.toml support for OIDC and server configuration

- Add toml crate dependency for TOML file parsing
- Create config module with Config struct to deserialize config.toml
- Support OIDC URLs (auth_url, token_url, redirect_url), credentials (client_id, client_secret)
- Make server host/port, database URL, and session secret configurable
- Create config.example.toml with all configuration options documented
- Update main.rs to load config.toml with fallback to environment variables
- Maintain backward compatibility with environment variable configuration

config.example.toml

@@ -0,0 +1,20 @@
+# OIDC Configuration
+[oidc]
+client_id = "your_client_id"
+client_secret = "your_client_secret"
+auth_url = "https://your-provider.com/auth"
+token_url = "https://your-provider.com/token"
+redirect_url = "http://localhost:3000/auth/callback"
+
+# Server Configuration
+[server]
+host = "127.0.0.1"
+port = 3000
+
+# Database Configuration
+[database]
+url = "sqlite:weight_tracker.db"
+
+# Session Configuration
+[session]
+secret = "your_secret_key_that_is_long_enough_so_the_library_does_not_complain"