Rename project to trailhead-path-bookmarker

- npm name: trailhead-path-bookmarker (package.json + lockfile)
- electron-builder appId: com.dbeuttel.trailhead-path-bookmarker
- AppUserModelId in main.js matches the new appId
- README title updated; userData path now %APPDATA%\Trailhead Path Bookmarker

Icon generator now emits both sizes:
- assets/tray-icon.png (32x32) for the system tray
- assets/app-icon.png (256x256) for electron-builder's win.icon, which
  rejects PNGs smaller than 256

electron/main.js

@@ -750,7 +750,7 @@ ipcMain.handle('set-popup-width', (_event, w) => {
 
 app.whenReady().then(() => {
   if (process.platform === 'win32') {
-    app.setAppUserModelId('com.dbeuttel.folder-bookmark-tray');
+    app.setAppUserModelId('com.dbeuttel.trailhead-path-bookmarker');
   }
   migrateConfigIfNeeded();
   pinned = !!readConfig().pinned;