Tool routes now include the modality prefix: /image/crop, /video/trim-video,
/audio/convert-audio, /document/merge-pdf, /data/csv-json. Old URLs
(e.g. /resize) redirect to the new modality-prefixed path automatically.
Routes are computed by prepending MODALITY_URL_SLUG to each tool's route
in the TOOLS array at module init. The App.tsx route changed from
/:toolId to /:modality/:toolId, with a LegacyToolRedirect fallback.
