feat: Add frontend support for server-side settings storage

[malhotra5]

This PR adds frontend support for the server-side settings storage feature introduced in #5594.

Changes

• Add Settings interface and API endpoints for loading/storing settings
• Create settings slice for Redux state management
• Add useSettings hook for easy access to settings functionality
• Update conversation initialization to use stored settings

This PR complements #5594 by providing the frontend implementation needed to persist user settings (including API keys) on the server side.

Testing

• Test loading settings from backend
• Test saving settings to backend
• Test settings persistence across sessions
• Test merging stored settings with conversation overrides

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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:b3ad9df-nikolaik   --name openhands-app-b3ad9df   docker.all-hands.dev/all-hands-ai/openhands:b3ad9df