Selecting a nested folder as a filter destination stored only the leaf
name, producing a Sieve fileinto action the server could not resolve
(e.g. "Foo" instead of "Inbox/Projects/Foo"). Add a getMailboxFullPath
helper that walks parentId and joins with "/", and route the filter
modal's move/copy dropdown through it so the emitted script references
the full path.
The helper guards against unknown ids, orphaned parents, and
self-referential cycles.
Closes#62.