feat: single-file project import/export (.ocp)

fflate zip container: project.json (serialized project incl. schema
version) + media files with a manifest. Export gathers all project
media (missing blobs warn, never abort) after save-flush; import
validates everything before any write, migrates older archives through
the storage migration registry, remaps media ids, and creates a new
project with rollback on failure. Entry points: projects page
import/export, editor header menu, export-project-archive action.
29 tests incl. full round-trip.

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