[TEST] add data sync

development.md

@@ -12,6 +12,7 @@ src/               main process (Node/Electron backend)
   ipc/              all ipcMain handlers
     index.js       handler registration, icon protocol, dialogs
     games.js       games service: run/stop state, wine resolution, open-with handoff
+    lansync.js     LAN sync handlers (identity, devices, re-roll, sync + progress)
   runner.js        spawns wine/proton (env, prefix, playtime, logs)
   library.js       apps registry (apps.json) + settings + playtime files
   wines.js         wine/proton build scan, registry, portable symlink healing
@@ -22,6 +23,8 @@ src/               main process (Node/Electron backend)
   paths.js         portable layout, FILES-relative path helpers, translation sync
   i18n.js          translation loading + {0} placeholder filling
   preload.js       contextBridge API exposed to the renderer
+  lansync.js       LAN sync: identity + pairing, hashing, HTTP receiver, UDP discovery
+  lansyncpush.js   the pushing side (createPusher factory, keeps helper state out)
   appicon.js       window/taskbar icon
 renderer/          UI (plain HTML/CSS/JS, no framework)
   app.js           views, modals, custom-select, all renderer logic
@@ -61,6 +64,9 @@ handlers in src/ipc/. Keep new APIs in that pattern.
 - CSS: reuse the existing variables/classes (.button, .field-label,
   .custom-select, form-card); spacing rhythm comes from --button-margin and
   the flex gap of the container, not per-element magic numbers.
+- LAN sync has two sides: the receiver lives in lansync.js, the sender in
+  lansyncpush.js and is built by createPusher(helpers) - several launcher
+  states share the module, so nothing may be kept in its module scope.
 - Tests are plain assert-by-print scripts (see test/): set PLWL_BUILD_DIR to
   a temp dir, check(), exit 1 on failure. Add one when fixing a bug.
 
@@ -69,5 +75,6 @@ handlers in src/ipc/. Keep new APIs in that pattern.
 ```
 node --check <changed js files>     # syntax
 node test/import.test.js            # ...and the other test files
+node test/lansync.test.js           # LAN sync contract (identity, pairing, mirror)
 npm start                           # click through the flow you touched
 ```