fix(core,fuse): refuse sync roots that overlap home, the mount or app data

Nothing stopped `/`, the home folder, the Proton Drive mount, the app's
own state or cache, or a folder nested in another root from becoming a
sync root, and a restore took a path from another machine's profile as
it was. One validator, `syncroot::problem`, now runs where roots are
added and restored, on paths resolved through symlinks with `..`
refused (B234).

crates/pdfs-core/src/lib.rs

@@ -23,6 +23,7 @@ pub mod service;
 pub mod shell;
 pub mod similar;
 pub mod syncignore;
+pub mod syncroot;
 pub mod takeout;
 pub mod tray;