[TEST] add data sync

README.md

@@ -47,6 +47,18 @@ Besides copy/move/leave, an app can be imported **from a wine prefix**: pick the
 
 The desktop entry (`MimeType` + `Exec %f`) makes PLWL show up in Dolphin/Nautilus under **Open With** for Windows executables (`.exe`, `.msi`, `.bat`, `.cmd`, `.lnk`). Opening a file that way shows a small popup in the launcher where you pick the **Wine/Proton version** and the **prefix** for this run — both default to the launcher's defaults, nothing is imported into the library. A second launcher instance hands the path to the running one and exits (same single-instance mechanism the shortcuts use).
 
+## LAN syncing
+
+Two PLWL installs on the same LAN can mirror each other's content - no cloud, no accounts. Open **LAN syncing** in the sidebar (under Settings):
+
+- **Device ID** - 24 characters, generated once per install (current timestamp + random characters) and re-rollable at any time.
+- **Sync code** - 8 characters, letters and digits, case-insensitive. A device only accepts a connection when the caller proves it knows *its own* code; each verification costs two seconds and a device may only have one in flight, so guessing over the LAN is pointless. Both the ID and the code can be re-rolled.
+- **Devices on your network** - other PLWL installs, found over UDP; a device shows up only while PLWL is running there, and disappears a few seconds after it is closed.
+
+Pick a device and choose what to push: **portable apps**, **apps imported from a prefix**, **Wine/Proton builds** and **prefixes**. The code has to be typed once - an accepted code is stored next to that device ID, so every later sync needs no typing. If the other side re-rolled its code, the stored one is forgotten and the launcher asks for the new one instead of failing silently.
+
+The **sending device is the source of truth**: files that differ are overwritten, missing ones are written, and anything living only on the receiving side inside the synced trees (`files/apps/`, `files/wines/`, `files/wineprefixes/`, `files/data/icons/`) is deleted. Files are compared with a fast, collision-resistant hash (BLAKE2b-512, SHA-512/SHA-256 fallback); **Force sync all files** skips the comparison and overwrites everything instead. Everything else stays local: settings, logs, playtime/last played, translations, and external (leave) apps - prefixes are always kept usable, so app entries pointing inside a prefix keep their prefix-relative paths and the receiving side never loses its own `Main` prefix.
+
 ## Wine & Proton support
 
 - **System Wine** — uses the `wine` binary from PATH.