feat(core,fuse,gui): show a sync issue on its node in the file browser

A refused change was only on the Sync page. A folder listing now carries
each node's issue (by local id, so a file not yet on Drive has it too),
and the browser's badge ranks it first, folders included, with the
issue's explanation as its tooltip. The event sampler publishes Files
when the set of issues changes, so the badge comes and goes with it.

docs/USER_GUIDE.md

@@ -169,6 +169,8 @@ The version you do not keep goes to the Trash, so a wrong choice can be undone.
 - **Retry** a failed upload from **Sync → Overview**, or with `pdfs sync retry`.
 - **Discard** a change Proton Drive refused from the same list, or with `pdfs sync discard`: it is
   undone here and never sent. Export a file first to keep your version.
+- A file or folder with a change Proton Drive refused carries a warning badge in **Files**; hover
+  it to see why.
 
 ## Photos