fix: revoke generated object URLs

[ifer47]

Summary

• revoke the temporary object URL used while generating image/video thumbnails
• create the vertd error-message blob URL when the dialog mounts and revoke it when the dialog unmounts

Why

URL.createObjectURL() keeps the underlying Blob/File data alive until the URL is revoked. The previous thumbnail path created one object URL per uploaded media file without revoking it, and the vertd error details link created a blob URL directly during render. Repeated conversions or repeated error dialog opens could retain unnecessary browser memory.

Testing

• bun run build with public env values from .env.example

Notes: bun run check currently fails on this clean checkout before reaching this change because generated Paraglide modules/env declarations and Node types are missing from the check setup. bun run lint also hits existing repository-wide Prettier/{@html} baseline issues.