Replace env-var-only LIBRARY_REMOTE configuration with disk-persisted
settings. The setting can now be edited in the dashboard Settings page,
with LIBRARY_REMOTE env var serving as initial default when no settings
file exists.
Changes:
- Add src/settings.rs for settings storage with JSON persistence
- Add src/api/settings.rs for settings API endpoints
- Update dashboard Settings page with editable library remote field
- Update library-unavailable component to link to Settings page
- Update documentation to recommend Settings page method
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