Set default offline LLM

[barun-saha]

When SlideDeck AI is run in offline mode, the Ollama model name has to be specified manually. An optional default model name may be provided to improve the user experience.

Introduce a new environment variable, DEFAULT_OFFLINE_MODEL_NAME. In the application, under the RUN_IN_OFFLINE_MODE block, add new code to retrieve the value of the env variable and, if defined or not empty, set the model name.

Tests

With default model name

1. Set DEFAULT_OFFLINE_MODEL_NAME=gemma3:1b in .env.
2. Run the app in offline mode.
3. The default model name should be visible in the app.
4. Create a PPT with text inputs to verify that it's working.

Without default model name

1. Remove DEFAULT_OFFLINE_MODEL_NAME from .env.
2. Run the app in offline mode.
3. No default model name should be visible in the app.
4. Set gemma3:1b as the model name.
5. Create a PPT with text inputs to verify that it's working.