fix(fuse): skip unreadable folders in sync, never evict local-only files

An unreadable subfolder, such as a container's root-owned cache, failed
every pass of its synced folder, so nothing else in the tree reached
Drive (#33, B196). The scan now leaves such a path out, together with an
entry that vanished mid-scan and a name that is not UTF-8, and the pass
drops everything under it from classification, the wipe guard and the
push pass's deletions. It is never read as deleted and never downloaded
into. An unreadable or absent root still fails the pass. The activity
feed names unreadable paths once each time the set changes.

Two related bugs on the way to online-only:

- The push pass read deletions from the raw baseline, so a synced path
  that later became ignored was trashed on Drive (B197).
- The switch evicted everything once the folder was idle, including
  ignored paths such as .git/, symlinks, unreadable folders and files
  changed after the pass, none of which are on Drive (B198). It now
  refuses and names the path.

Closes #33

docs/USER_GUIDE.md

@@ -96,7 +96,9 @@ app and in the web app.
 Each synced folder can also be switched to **Online only**. The local copy is replaced by a mount
 of the folder, so the files stay visible but take no disk space until opened. Switch with the
 **Synced / Online only** switch on the folder's card, or `pdfs sync mode <id> ondemand` and back
-with `pdfs sync mode <id> mirror`. `pdfs locations` lists every local place the service occupies.
+with `pdfs sync mode <id> mirror`. The switch waits until everything in the folder is on Proton
+Drive, and refuses while the folder holds something that never goes there: an ignored path such
+as `.git/`, a symlink, or a folder you may not read. The message names it. `pdfs locations` lists every local place the service occupies.
 
 ### Moving between synced folders and My files
 
@@ -146,6 +148,9 @@ places, and both apply:
 Rules are re-read on every pass. Ignoring never deletes anything: a file that becomes ignored is
 simply no longer tracked, and its copy on Proton Drive stays.
 
+A folder you may not read, such as a cache a container created as root, is skipped the same way,
+and the activity feed names it once. Add it to `.pdfsignore` to stop that message.
+
 ## Changes waiting for Proton Drive
 
 A change you made in the Proton Drive folder exists only on this computer until it reaches