fix: display only real sync errors in error count and list

- Added CountErroredChanges() method to count only pending changes with LastError
- Previously, error count included all pending changes, not just failed ones
- Added /api/sync/info endpoint with proper error count and error list
- Added sync info modal to display sync status, error count, and error details
- Made sync status indicators clickable to open the modal
- Fixed disconnect between "Error count: 4" and "No errors" in the list

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

cmd/server/main.go

@@ -625,6 +625,7 @@ func setupRouter(db *gorm.DB, cfg *config.Config, local *localdb.LocalDB, dbUser
 		syncAPI := api.Group("/sync")
 		{
 			syncAPI.GET("/status", syncHandler.GetStatus)
+			syncAPI.GET("/info", syncHandler.GetInfo)
 			syncAPI.POST("/components", syncHandler.SyncComponents)
 			syncAPI.POST("/pricelists", syncHandler.SyncPricelists)
 			syncAPI.POST("/all", syncHandler.SyncAll)