docs: describe local-first changes and sync issues for users

The user guide said only writes were queued. Since 3.0.0 creating,
renaming and deleting through the mount return at once too, and changes
Drive refuses become sync issues. Add a section on changes waiting for
Proton Drive: retry, export, discard, and the two cases that land under
another name or place instead of failing.

Troubleshooting covers changes that have not reached other devices yet,
`pdfs sync issues` and the local_first fallback, and names the real
error for symlinks and hard links (EPERM, not EOPNOTSUPP). CONFIGURATION
lists MemoryHigh and the test variables; CLI notes that `sync export`
takes a folder.

docs/USER_GUIDE.md

@@ -7,6 +7,7 @@ How Proton Drive for Linux works and how to use it day to day. For installation
 - [My files](#my-files)
 - [Keeping files on this computer](#keeping-files-on-this-computer)
 - [Syncing local folders](#syncing-local-folders)
+- [Changes waiting for Proton Drive](#changes-waiting-for-proton-drive)
 - [Conflicts](#conflicts)
 - [Pausing and limiting sync](#pausing-and-limiting-sync)
 - [Photos](#photos)
@@ -33,7 +34,7 @@ the service to do things over a local socket:
 
 Your Drive, your file names and your photos are end-to-end encrypted by Proton. On this computer,
 the service must decrypt what it shows you, and it keeps a decrypted cache and index on disk. Use
-full-disk or home-directory encryption. [ARCHITECTURE.md §8](ARCHITECTURE.md#8-threat-model-what-this-client-writes-to-disk-in-plaintext)
+full-disk or home-directory encryption. [ARCHITECTURE.md §10](ARCHITECTURE.md#10-threat-model-what-this-client-writes-to-disk-in-plaintext)
 lists exactly what is stored.
 
 The app uses these words consistently, and so does this guide:
@@ -54,9 +55,11 @@ downloads only when a program reads it:
 
 - **Reads fetch only what is needed.** A video player that seeks, or a tool that reads a file
   header, downloads only those parts. Media streams while it plays.
-- **Writes are safe offline.** When a program closes a file it wrote, the service stores the new
-  content on disk and queues the upload. The queue survives network loss, restarts and crashes.
-  `pdfs sync queue` lists what has not reached Proton Drive yet.
+- **Changes work at local speed.** Creating, renaming, moving and deleting files and folders
+  returns at once, online or offline. When a program closes a file it wrote, the new content is
+  stored on disk. Every change is queued and sent to Proton Drive in the background; the changes
+  to one file are sent in the order you made them. The queue survives network loss, restarts and
+  crashes. See [Changes waiting for Proton Drive](#changes-waiting-for-proton-drive).
 - **Recently read content is cached** on disk up to the cache size limit (5 GiB by default) and
   evicted oldest first.
 - **Items shared with you** appear under `Shared with me/` inside the Proton Drive folder. Shares
@@ -143,6 +146,36 @@ 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.
 
+## Changes waiting for Proton Drive
+
+A change you made in the Proton Drive folder exists only on this computer until it reaches
+Proton Drive. `pdfs status`, the tray and **Sync → Overview** count what is still waiting, and
+`pdfs sync queue` lists it.
+
+Sometimes Proton Drive refuses a change: the storage is full, a share was taken away, the folder
+is gone, the folder holds too many items, or Drive does not accept a name. The change then needs
+your attention. It shows at once in **Sync → Overview**, in the tray, and in `pdfs sync issues`,
+with what it means and what you can do. A file or folder with such a change carries a warning
+badge in **Files**; hover it to see why.
+
+Nothing is deleted. The change stays queued and is retried. You can:
+
+- **Retry** it now, from the same list or with `pdfs sync retry`, for example after freeing space.
+- **Export** a file's content to keep your version (`pdfs sync export <id> <file>`).
+- **Discard** the change (`pdfs sync discard <id>`). It is undone here and never sent: a new file
+  or folder goes away, anything else goes back to what Proton Drive has. Export first if you want
+  to keep the content.
+
+Two cases are not refused, and the activity log records both:
+
+- A new file or folder whose name is already taken on Proton Drive by something this computer had
+  not seen yet lands under a `(sync-conflict …)` name, as described below.
+- A new file whose folder was deleted on another device meanwhile lands at the top of My files,
+  so its content is not lost.
+
+Before you shut down a computer for good, check that nothing is waiting: see
+[RECOVERY.md](RECOVERY.md#lost-with-the-machine).
+
 ## Conflicts
 
 When the same file changed on this computer and somewhere else since the last sync, neither side
@@ -166,11 +199,6 @@ The version you do not keep goes to the Trash, so a wrong choice can be undone.
   reading files keeps working. `pdfs sync resume` continues.
 - **Limit bandwidth** under **Preferences → General → Network**, or with
   `pdfs sync limit --up 2M --down 10M`. `0` removes a limit.
-- **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