Report and retry Docker resources remove_project could not delete

remove_project_volumes always returned Ok(()) regardless of what actually
happened, making the `if let Err(e)` guarding it at every call site dead
code. remove_project then dropped the project record unconditionally, so a
volume, image or container that failed to delete became permanently
unreachable — confirmed against a real orphaned volume pair found in the
wild (fixes #31).

remove_project_volumes/remove_snapshot_image/remove_container now report
what they could not remove (treating "already gone" as success rather than
a leftover), remove_project surfaces this to the user via a toast, and
before dropping the project record it writes a pending-cleanup record that
startup housekeeping retries automatically on the next launch.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FGjXq6fqtAFHdbhk4f3PfZ

app/src-tauri/src/storage/mod.rs

@@ -1,4 +1,5 @@
 pub mod migration_store;
+pub mod pending_cleanup;
 pub mod projects_store;
 pub mod secure;
 pub mod settings_store;