Support custom token mapping in plugin

[nathanacurtis]

Problem

The CLI supports custom token mappings — users can provide a JSON file that maps Figma token IDs to custom names, enabling the CUSTOM token profile to emit user-defined token names. However, the Figma plugin has no way to accept or apply a custom token mapping. Users running Specs entirely through the plugin cannot use the CUSTOM token profile.

Solution

Add a mechanism in the plugin for users to supply a custom token mapping (via paste, file upload, or a stored reference). The mapping should be applied before processing so the CUSTOM token profile works identically in both plugin and CLI environments. Unmatched mapping IDs should surface as warnings in the plugin UI.

Acceptance criteria

• Users can provide a custom token mapping through the plugin UI
• The CUSTOM token profile produces the same output in the plugin as in the CLI
• Unmatched mapping IDs are reported as warnings
• Mapping optionally persists across plugin sessions

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Implementation details are tracked internally.