Add URL parameter support for automatic codelist loading

[milanwiedemann]

Enable the Shiny app to accept URL parameters (e.g., ?codelist=opensafely/anxiety-disorders/6aef605a) to automatically load codelists when the app starts, improving integration with OpenCodelists.org and user experience.

Currently, users must manually copy-paste codelist slugs into the app. Direct linking from http://opencodelists.org/ would imporove the workflow and make the tool more accessible for researchers exploring code usage patterns of codelists.

Proposed Implementation

• URL parameter parsing: Use parseQueryString() to extract codelist parameter from URL
• Automatic text input population: Update codelist_slug text input using updateTextInput()
• Programmatic button trigger: Automatically trigger the "Load codelist" action button

Technical approach

• Parse URL parameters in server.R using session$clientData$url_search
• Check for codelist parameter on app initialisation
• If present, populate the text input and simulate button click
• Handle error cases gracefully (invalid codelist slugs, network issues)

Future enhancements

• Support multiple parameters (dataset, date_range)
• Validation of codelist compatibility with selected dataset

Acceptance Criteria

• App accepts ?codelist= URL parameter
• Codelist automatically loads on app startup when parameter present
• Error handling for invalid codelist slugs
• Maintains existing functionality when no parameters provided
• Documentation updated with URL parameter usage examples