Implement LLM client and normalization settings

- Added LLMClient class for handling requests to LLM API, including methods for listing models and generating completions.
- Introduced LLMConfiguration dataclass for managing LLM configuration settings.
- Created normalization_settings module to manage normalization configurations and environment variable overrides.
- Developed JavaScript functionality for the settings interface, including model fetching and preview generation for LLM and normalization.
- Enhanced user experience with status messages and error handling in the settings UI.

.env.example

@@ -25,3 +25,14 @@ ABOGEN_TEMP_DIR=./storage/tmp
 #   id -g   # GID
 ABOGEN_UID=1000
 ABOGEN_GID=1000
+
+# Optional: Seed the web UI with working defaults for the LLM-powered
+# text normalization features. Leave these blank to configure everything
+# from the Settings page.
+ABOGEN_LLM_BASE_URL=https://localhost:11434/v1
+ABOGEN_LLM_API_KEY=ollama
+ABOGEN_LLM_MODEL=llama3.1:8b
+ABOGEN_LLM_TIMEOUT=45
+ABOGEN_LLM_CONTEXT_MODE=paragraph
+# For custom prompts, keep the text on a single line or escape newlines.
+#ABOGEN_LLM_PROMPT=You are assisting with audiobook preparation. Rewrite {{sentence}} for narration.