feat: migrate settings storage from localStorage to server API

[tofarr]

This PR migrates the frontend settings storage from localStorage to the server-side API endpoints.

Changes:

• Removed localStorage-based settings management
• Updated Settings type to make LLM_API_KEY nullable
• Converted getSettings and saveSettings to async functions
• Added proper error handling for API requests
• Preserved API key management security behavior

Note: Components using these functions will need to be updated to handle the async nature of the new implementation.

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:7063ff3-nikolaik   --name openhands-app-7063ff3   docker.all-hands.dev/all-hands-ai/openhands:7063ff3