fs(images): clean up a project's qualified image namespace directory on project deletion

[qnbs]

Problem

FsProjectStore.deleteProject() removes a project's own directory (appData/projects/<id>/, including its binder assets and codex), but does not know about the project-qualified image namespace directory introduced in #708-A (appData/images/<projectNamespaceSegment>/, keyed by a SHA-256 digest of the project id, not the project id's own directory name). Deleting a project therefore leaves every qualified image for that project as an orphaned, unreachable directory under appData/images/.

This is a disk-space/storage-hygiene leak, not a correctness or security issue: the images are unreachable (no code path can construct their namespace segment without the original project id), not incorrectly exposed to another project.

Invariant

Deleting a project should also remove its project-qualified image namespace directory, while preserving the existing legacy-flat-image safety rules established in #708-A (ownership-marker-gated deletion of the unqualified/pre-qualification appData/images/*.png files, and the .legacy-owner marker itself).

Scope

• FsProjectStore.deleteProjectUnlocked() (or a sibling helper in FsAssetStore) computes the same project-namespace segment saveImage/getImage/deleteImage use and removes that directory as part of project deletion.
• Must not touch the separate legacy-flat-image ownership logic (.legacy-owner marker, appData/images/*.png flat files) -- those follow their own, already-implemented preserve-first rules.
• Failure to remove the namespace directory should be logged and non-fatal to the rest of project deletion (matching this method's existing error-handling style), not silently swallowed without a trace.

Non-goals

• No changes to the IndexedDB backend (single-project-at-a-time architecture; this is filesystem-backend-specific).
• No changes to the legacy-image ownership-marker semantics themselves.
• No broader project-lifecycle/cleanup redesign.

Dependencies

Builds on #708-A's project-qualified image namespace (projectNamespaceSegment() in services/fs/assetFsStore.ts).

Regression test

A test proving: save an image for project A, delete project A, assert the qualified image directory for A no longer exists, while a legacy flat image already owned by a different project is left untouched.