Custom variables currently exist at the project level, meaning users have to define them again for every project/report. Want the ability to set variables once at the workspace level so they're prefilled and standardized across all projects and reports in the workspace.

Current behavior: Custom variables are scoped to a single project/report. Common values like client name must be manually re-entered for each new project.

Desired behavior: Workspace-level variables that cascade down to all projects, with project-level variables still available for overrides or project-specific values.

Acceptance criteria:

Workspace admins can define custom variables at the workspace level

Workspace variables are automatically available in every project/report

Project-level variables can override workspace-level variables when needed

Updating a workspace variable propagates to all reports that reference it