feat(config): add configuration management for Yarr TUI app

yarr.toml.example

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+# Yarr Configuration File
+# Copy this file to one of the following locations:
+# - ./yarr.toml (current directory)
+# - ~/.config/yarr/config.toml (user config directory)
+
+[sonarr]
+# Sonarr server URL (required)
+# Example: "http://localhost:8989" or "https://sonarr.example.com"
+url = "http://localhost:8989"
+
+# Sonarr API key (required)
+# You can find this in Sonarr under Settings > General > Security > API Key
+api_key = "your-api-key-here"
+
+# Environment variables can also be used:
+# YARR_SONARR_URL="http://localhost:8989"
+# YARR_SONARR_API_KEY="your-api-key-here"
+#
+# Command line arguments take highest priority:
+# yarr --sonarr-url="http://localhost:8989" --sonarr-api-key="your-key"